Re: OT: Hybrid cars
guy keren writes: > actually, driving at 70-80kmh is usually MUCH MORE fuel-efficient then > driving at 110kmh, in most cars and under most road conditions... This may *still* be true for many cars on Israeli roads, but it should not be regarded as some law of nature or engineering, and it is not a universal constant. Chances are, it no longer holds for many newer models (or for any: I'd say the sweet spot was around 90km/h 10-15 years ago for family-sized cars). The exact number is less important than the dynamics, in my mind. So let's consider how it evolves. I'll skip the basic physics (unless someone requests), but you can design a car for particular requirements, including for most efficient speed. Different engines are designed, and can be tuned differently by default, depending on the market. For instance, cars sold here are not identical to those sold in Europe. They drive *much* faster on European highways than we drive in Israel. Typical highway speed limits are 120-130 km/h, 110 km/h on smaller roads. Their requirements are different. Actually, our highway speed limits are increasing, albeit slowly: 100-110 km/h is quite common today, as opposed to the universal 90 km/h just a few years ago. This may already be reflected in how engines are tuned here (and car manufacturers cannot keep developments in check because of a small and "retarded" market such as Israel, so we have to accommodate newer models), but you certainly need to talk to a qualified mechanic and not to me. I do not keep records or statistics, but my unscientific observations lead me to believe that my 6 year old Passat is more efficient at 110km/h on Highway 1 or 2 than at 80-90. But it may be very different from most contemporary Mazda 3's and Corollas and what not, e.g., the 6th gear must be somewhat helpful at higher speeds, it may use a leaner air-fuel ratio than many other engines, etc. Most importantly, its engine power vs. aerodynamics may be very different (I promised to skip the physics, but aerodynamics are crucial). Generally, smaller cars are often intended for urban driving - shorter, slower rides, easy parking, etc. It makes sense to optimize for lower speeds, frequent gear shifts (at low gear), shorter gear ratios, etc. Note that you cannot optimize much for too low speeds (basic physics again). An important relevant consideration when discussing the dynamics of this is that it makes more sense to improve highway consumption than urban one to improve the mixed number, since you typically drive longer distances on highways and you quote average mileage that is skewed towards highways. E.g., if your daily mix when driving to the city to work and back is 100km on the highway (at 20km/l) and 10km in the city (at 10km/l) then you use 6l over your trip (5 on the road, 1 in the city). Improving highway consumption by 10% will reduce your daily total to ~5.5l. Improving your city consumption by 10% will reduce your total to ~5.9l. If you improve highway by 10% but screw up the city part by 10% you will still be better off at less than 5.6l. If you improve the city part by 10% but your highway becomes 10% worse you'll be at almost 6.5l. >From this it follows that as your roads and cars get better and faster (and safer) it makes a lot of sense to optimize for higher road speeds. If you can drive on Road 2 at 110km/h rather than 90km/h (my example above roughly corresponds to commuting from Hadera to TA or to Haifa), it makes sense to optimize for highway. Even if you buy such a car and use it mostly inside the city it is still not bad since you are not in an efficient regime anyway and much less can be done under 50km/h. (Well, getting a small, light car in this case will make sense, and screw aerodynamics). -- Oleg Goldshmidt | p...@goldshmidt.org ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
On 09/16/2013 10:28 AM, Oleg Goldshmidt wrote: guy keren writes: actually, driving at 70-80kmh is usually MUCH MORE fuel-efficient then driving at 110kmh, in most cars and under most road conditions... This may *still* be true for many cars on Israeli roads, but it should not be regarded as some law of nature or engineering, and it is not a universal constant. Chances are, it no longer holds for many newer models (or for any: I'd say the sweet spot was around 90km/h 10-15 years ago for family-sized cars). The exact number is less important than the dynamics, in my mind. So let's consider how it evolves. instead of going into theories - does your car have a fuel consumption computer? if it does - please perform an experiment: reset the counter before your next two drives on the highway. on the first drive - drive at 110km/h. on the second drive - go at 80km/h. perform the two tests on a flat area outside the city (i.e. reset the counter when you're outside of the city). then come back with the results. i did this in the past, both on the Prius (2008 model) and on a renault megan (2000) - the difference was noticeable, in favor of the slower speed. when you perform a similar experiment in europe (again, with a computer that measures fuel consumption), with a european car - then we can figure out whether these theories agree with reality. --guy - in theory - there is no difference between theory and practice. in practice - there is. ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
guy keren writes: > instead of going into theories - does your car have a fuel consumption > computer? Yes, it does, that's how I know that it is more efficient at higher speeds. I made a point to say that I never did systematic observations or statistical analyses, just watched the numbers occasionally out of curiousity. In effect it was exactly the kind of experiment you suggested. My car is different from yours, that's all. Your Prius, in particular, may use relatively more battery at lower highway speeds giving you momentarily better numbers (I don't know that, I am guessing). I assume it is not a plug-in, so at some point it will consume some fuel to recharge the battery and your numbers may be momentarily worse. I assume it is smart enough to do it when the engine is not under load and when you are in a lousy regime (in a traffic jam, etc.). This would be smart on two levels: a) charge the battery when you have spare capacity; b) this regime will improve the average numbers, exactly as I showed in the previous email. To emphasize again: all of the above regarding what your Prius may or may not do is guesswork. Not so unreasonable guesswork, I hope. But even if it is basically correct, it also may be just a component in the overall picture. My car has a significantly larger engine, probably uses a different AFR, definitely a completely different gearbox (and quite probably lower RPMs at higher speeds), different aerodynamics. It is not reasonable to expect a particular derived characteristic (optimal speed for fuel consumption) to be similar for suc different models. Even the markets for which the cars were designed by the manufacturers are completely different: Prius's target market is definitely closer to California than to Europe, while Passats are not very popular in the US but common in the Old World. Guess what: Americans drive much slower on average (highway speed limits between 55mph and 65mph). This could easily affect design decisions. [Again: no, I did not watch over the shoulders of Toyota or VW engineers.] -- Oleg Goldshmidt | p...@goldshmidt.org ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
2013/9/16 Oleg Goldshmidt : > guy keren writes: > >> instead of going into theories - does your car have a fuel consumption >> computer? > > Yes, it does, that's how I know that it is more efficient at higher > speeds. I made a point to say that I never did systematic observations > or statistical analyses, just watched the numbers occasionally out of > curiousity. In effect it was exactly the kind of experiment you > suggested. > > My car is different from yours, that's all. Your Prius, in particular, > may use relatively more battery at lower highway speeds giving you > momentarily better numbers (I don't know that, I am guessing). I assume > it is not a plug-in, so at some point it will consume some fuel to > recharge the battery and your numbers may be momentarily worse. I assume > it is smart enough to do it when the engine is not under load and when > you are in a lousy regime (in a traffic jam, etc.). This would be smart > on two levels: a) charge the battery when you have spare capacity; b) > this regime will improve the average numbers, exactly as I showed in the > previous email. > > To emphasize again: all of the above regarding what your Prius may or > may not do is guesswork. Not so unreasonable guesswork, I hope. But even > if it is basically correct, it also may be just a component in the > overall picture. My car has a significantly larger engine, probably uses > a different AFR, definitely a completely different gearbox (and quite > probably lower RPMs at higher speeds), different aerodynamics. It is not > reasonable to expect a particular derived characteristic (optimal speed > for fuel consumption) to be similar for suc different models. Even the > markets for which the cars were designed by the manufacturers are > completely different: Prius's target market is definitely closer to > California than to Europe, while Passats are not very popular in the US > but common in the Old World. Guess what: Americans drive much slower on > average (highway speed limits between 55mph and 65mph). This could > easily affect design decisions. [Again: no, I did not watch over the > shoulders of Toyota or VW engineers.] > > -- > Oleg Goldshmidt | p...@goldshmidt.org > > ___ > Linux-il mailing list > Linux-il@cs.huji.ac.il > http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il European lawmakers when talking about pollution and efficiency generally want to lower maximum speeds since it entails less pollution and more fuel efficiency... Results obviously may differ per car. Regards, Eliyahu - אליהו ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
"E.S. Rosenberg" writes: > European lawmakers when talking about pollution and efficiency > generally want to lower maximum speeds since it entails less pollution > and more fuel efficiency... Let's say, for the sake of the argument, that all cars in France are tuned to 100km/h as the most economical speed. The highway speed limit in France is, IIRC, 130km/h. This basically means that unless there is a problem (accident, poor visibility, rain, whatever) everybody is *supposed* to drive at 130km/h on highways. Driving too slow is a violation in every jurisdiction I know. [Well, the French used to drive a lot faster, I thin they increased the fines now...] So you are supposed to drive on a highway faster than your optimal speed. How can you make people drive at the optimal speed? Two solutions: lower the speed limit (and reap more fines, maybe, at the expense of a lot more time being wasted on the roads if your speed is reduced by 20-30%, and bias towards older, actually more polluting cars), or improve aerodynamics of cars so that their optimum is closer to the limit. Hmmm... A dilemma... Now, consider the same situation, same cars, but your current speed limit is 100 or 110 km/h... There are reasons why, despite the screams from various influential, well-meaning but not very deeply thinking quarters, our highway speed limits have risen from 90 to 100-110 in many cases. -- Oleg Goldshmidt | p...@goldshmidt.org ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
On Mon, Sep 16, 2013, guy keren wrote about "Re: OT: Hybrid cars": > regarding the hybrid toyota yaris - i've no idea, as i don't know > anyone that owns this car. I had both a Toyota Prius and hybrid Yaris, and can share these numbers: I had a Chevrolet Cruze (a typical family-sized car) and was doing on my usual route (partly city, partly highway, partly climbing mountains, averaged over a few thousand kilometers) was 10.2L / 100km. When I switched to a Toyota Prius, on exactly the same route, my average went down to 5.6L / 100km. You can calculate yourself home much money this case save - likely not enough to justify the Prius's being 40,000 shekel more expensive then the Chevy. Then I switched routes (the new route includes more highway time), and got a new car, the tiny Nissan Micra Eco, which is known for its low fuel consumption for a non-hybrid car. This did on average (on the new route) 6.2L / 100km. Lastly, I switched to a hybrid Toyota Yaris, and got 5.2L / 100km. Conclusion? The hybrid Yaris is the most fuel-efficent car I ever owned. But the moeny saving - about 1L per 100km (or about 80 shekels per 1000km) will never repay the 30,000 shekels the Yaris costs more than the Micra. Of course, the Yaris is better than the Micra in almost every other thing (most importantly, in crash-test scores). -- Nadav Har'El| Monday, Sep 16 2013, 12 Tishri 5774 n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |error compiling committee.c: too many http://nadav.harel.org.il |arguments to function ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
Don't forget to put into the equation the environmental saving and tree hugging fill-good. What I'm getting to is that the new type of cars should be priced on a national level (reducing buyer's tax) which is a political issue. Simple beans counting will not favor them at all. According to Nadav, his yaris puts 5.2L/100km. We have a 2009 Ford Mondeo which we drive as it goes (i.e. pedal to the metal) and the trunk is always full with junk and it gives about 11L/100km, mainly in the city. Comparing 11/100 to 5/100, the big saving is 780nis per month (7.8nis/1L) based on 20,000km yearly. What I'm getting to, is that fuel consumption shouldn't be your first consideration. First consideration is find a car which fits your needs. Moish On 16/09/2013 15:42, Nadav Har'El wrote: On Mon, Sep 16, 2013, guy keren wrote about "Re: OT: Hybrid cars": regarding the hybrid toyota yaris - i've no idea, as i don't know anyone that owns this car. I had both a Toyota Prius and hybrid Yaris, and can share these numbers: I had a Chevrolet Cruze (a typical family-sized car) and was doing on my usual route (partly city, partly highway, partly climbing mountains, averaged over a few thousand kilometers) was 10.2L / 100km. When I switched to a Toyota Prius, on exactly the same route, my average went down to 5.6L / 100km. You can calculate yourself home much money this case save - likely not enough to justify the Prius's being 40,000 shekel more expensive then the Chevy. Then I switched routes (the new route includes more highway time), and got a new car, the tiny Nissan Micra Eco, which is known for its low fuel consumption for a non-hybrid car. This did on average (on the new route) 6.2L / 100km. Lastly, I switched to a hybrid Toyota Yaris, and got 5.2L / 100km. Conclusion? The hybrid Yaris is the most fuel-efficent car I ever owned. But the moeny saving - about 1L per 100km (or about 80 shekels per 1000km) will never repay the 30,000 shekels the Yaris costs more than the Micra. Of course, the Yaris is better than the Micra in almost every other thing (most importantly, in crash-test scores). -- Moish ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Announce: OSv, a new open-source operating system for virtual machines
Hi, today we've made the first release of OSv, a new operating system for running applications on virtual machines. OSv is free software, released under the BSD license, and you can find it in https://github.com/cloudius-systems/osv and http://www.osv.io. There is also a mailing list: osv-...@googlegroups.com. These days, most applications running on virtual machines in the cloud run on top of Linux. We all love Linux, but as an all-encompassing operating system for everything from phones to supercomputers, Linux was never really designed for virtual machines; It is big and complex, and it offers features (such as multi-user and multi-process) which are today made redundant by the hypervisor and slow it down. Linux's APIs are many times set in stone by decades of legacy code. All these cost in application performance, and make it harder to innovate. This is why we developed OSv, a new operating system designed to run a single application on a virtual machine. As it runs a single application there is no need for kernel-userspace isolation, reducing context switch costs and unnecessary copying. A design from scratch allowed us to experiment with new ideas like lock-free mutexes (solving the Lock-Holder Preemption problem that plagues operating systems on virtual machines), extremely fast context switches, Van Jacobson's network channels (see http://www.lemis.com/grog/Documentation/vj/lca06vj.pdf), and more. Also, OSv is released under the more permissive BSD license (not GPL like Linux), is tiny compared to Linux, and takes less than one second to boot and start the user's application. OSv can run ordinary Linux shared objects, such as, for example, an unmodified JVM (e.g., OpenJDK) executable, and of course on that you can run any application written in Java, JRuby , Clojure, or any other JVM language. Even at this early stage of OSv's development, OSv can already successfully run several interesting workloads such as Netperf, Memcached, Cassandra and SpecJVM - and usually match or even beat Linux's performance. Another refreshing feature of OSv is that is written in C++. It's been 40 years since Unix was (re)written in C, and the time has come for something better. C++ is not about writing super-complex type hierarchies (as some people might have you believe). Rather, it allowed us to write shorter code with less boiler-plate repetition and less chances for bugs. It allowed us to more easily reuse quality code and data structures. And using newly standardized C++11 features, we were able to write safe concurrent code with standard language features instead of processor-specific hacks. And all of this with zero performance overheads - most of C++'s features, most notably templates, are compile-time features which result in no run-time overhead compared to C code. OSv was developed by Cloudius Systems, a small Israeli startup led by Dor Laor and Avi Kivity (of KVM fame) but it is an open-source project - developed since its inception on github (https://github.com/cloudius-systems/osv), and released under the BSD license. We would like to take this opportunity to invite everyone to use OSv, and to help drive its development forward. OSv is a fantastic playground for kernel developers, and also for people involved in cloud development, devops, and so on. Tell us what your dream VM operating system will do, and maybe your dream will come true :-) Maybe you can even help us make that dream come true. If you want to try OSv, check out the announcement and usage instructions on the OSv mailing list: https://groups.google.com/forum/#!topic/osv-dev/enqdqN2A0as -- Nadav Har'El| Monday, Sep 16 2013, 13 Tishri 5774 n...@math.technion.ac.il |- Phone +972-523-790466, ICQ 13349191 |A computer once beat me at chess, but it http://nadav.harel.org.il |was no match for me at kickboxing. ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
On 09/16/2013 11:21 AM, Oleg Goldshmidt wrote: guy keren writes: instead of going into theories - does your car have a fuel consumption computer? Yes, it does, that's how I know that it is more efficient at higher speeds. I made a point to say that I never did systematic observations or statistical analyses, just watched the numbers occasionally out of curiousity. In effect it was exactly the kind of experiment you suggested. "watching the numbers occasionally" is not a proper experiment. you need to reset the computer before you start the "drive under test", and check the value after - and the length should be enough to even out the fluctuations. My car is different from yours, that's all. Your Prius, in particular, may use relatively more battery at lower highway speeds giving you momentarily better numbers (I don't know that, I am guessing). I assume it is not a plug-in, so at some point it will consume some fuel to recharge the battery and your numbers may be momentarily worse. I assume it is smart enough to do it when the engine is not under load and when you are in a lousy regime (in a traffic jam, etc.). This would be smart on two levels: a) charge the battery when you have spare capacity; b) this regime will improve the average numbers, exactly as I showed in the previous email. the experiments i performed were over lengthy periods of time. the numbers i reported in another mail were taken by reseting the counter every time after i refuel the car - and i usually refuel it in the same gas station. i also did not take into account periods in which i performed long out-of-town drives on road 2 or similar roads. one thing to note - the car uses more then just fuel to recharge the battery. every time i leave the accelerator (e.g. when coming to a traffic light, or due to getting too close to a car in front of me) - the battery is being recharged. without this mechanism, the car couldn't have been able to go at 20km/l in "accordion" traffic-jams (i tested this under a 15-minutes jam - that's the longest i encountered so far. and the battery's charge level went up and down several times during this period - implying the car could have supported the same level of fuel consumption even if the traffic jam lasted much longer). however, the car is able to sustain a 20km/l consumption rate also when going at a speed of 110km/h on road 2. it's just that at 80 - it could get even better consumption. without you giving more exact numbers and how exactly you measured them - i don't think we can make any comparisons. arguing about fuel consumption *reality* using theoretical guesswork is, in my opinion, pointless. To emphasize again: all of the above regarding what your Prius may or may not do is guesswork. Not so unreasonable guesswork, I hope. But even if it is basically correct, it also may be just a component in the overall picture. My car has a significantly larger engine, probably uses a different AFR, definitely a completely different gearbox (and quite probably lower RPMs at higher speeds), different aerodynamics. It is not reasonable to expect a particular derived characteristic (optimal speed for fuel consumption) to be similar for suc different models. Even the markets for which the cars were designed by the manufacturers are completely different: Prius's target market is definitely closer to California than to Europe, while Passats are not very popular in the US but common in the Old World. Guess what: Americans drive much slower on average (highway speed limits between 55mph and 65mph). This could easily affect design decisions. [Again: no, I did not watch over the shoulders of Toyota or VW engineers.] ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
since most cars are "good enough for me" - i went with the fuel saving and the "feel good" solution. i wouldn't have done that if i didn't get it for a very cheap price :0 "a car that fits your needs" - any car that has 4 wheels, gets from here to there, and doesn't break often - and has at lease the amount of space i need. --guy On 09/16/2013 07:50 PM, Moish wrote: Don't forget to put into the equation the environmental saving and tree hugging fill-good. What I'm getting to is that the new type of cars should be priced on a national level (reducing buyer's tax) which is a political issue. Simple beans counting will not favor them at all. According to Nadav, his yaris puts 5.2L/100km. We have a 2009 Ford Mondeo which we drive as it goes (i.e. pedal to the metal) and the trunk is always full with junk and it gives about 11L/100km, mainly in the city. Comparing 11/100 to 5/100, the big saving is 780nis per month (7.8nis/1L) based on 20,000km yearly. What I'm getting to, is that fuel consumption shouldn't be your first consideration. First consideration is find a car which fits your needs. Moish On 16/09/2013 15:42, Nadav Har'El wrote: On Mon, Sep 16, 2013, guy keren wrote about "Re: OT: Hybrid cars": regarding the hybrid toyota yaris - i've no idea, as i don't know anyone that owns this car. I had both a Toyota Prius and hybrid Yaris, and can share these numbers: I had a Chevrolet Cruze (a typical family-sized car) and was doing on my usual route (partly city, partly highway, partly climbing mountains, averaged over a few thousand kilometers) was 10.2L / 100km. When I switched to a Toyota Prius, on exactly the same route, my average went down to 5.6L / 100km. You can calculate yourself home much money this case save - likely not enough to justify the Prius's being 40,000 shekel more expensive then the Chevy. Then I switched routes (the new route includes more highway time), and got a new car, the tiny Nissan Micra Eco, which is known for its low fuel consumption for a non-hybrid car. This did on average (on the new route) 6.2L / 100km. Lastly, I switched to a hybrid Toyota Yaris, and got 5.2L / 100km. Conclusion? The hybrid Yaris is the most fuel-efficent car I ever owned. But the moeny saving - about 1L per 100km (or about 80 shekels per 1000km) will never repay the 30,000 shekels the Yaris costs more than the Micra. Of course, the Yaris is better than the Micra in almost every other thing (most importantly, in crash-test scores). -- Moish ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
guy keren writes: > "watching the numbers occasionally" is not a proper experiment. you > need to reset the computer before you start the "drive under test", > and check the value after - and the length should be enough to even > out the fluctuations. I don't know what your car shows you. Mine directly shows the fuel consumption at the moment (that jumps around) and a running average over some period of time that is updated every few seconds. I don't remember what the averaging period is exactly, but if you drive at a steady speed for a while (I have cruise control, too) you will get a pretty stable number on the screen. These are two different screens on the dashboard that I can switch between with a button on the steering wheel. The computer screen is right next to the speedometer, so I can watch the speed (even without cruise control) and fuel consumption simultaneously. In principle, I think there is another screen that reports your running average speed, but I don't think I used it for this purpose. I don't know what you mean by "resetting" the computer. I assume you reset the "trip distance" counter. I don't even need it to watch the fuel consumption numbers. It sounds like you took a trip with a full tank, guesstimated your average velocity, and topped the tank again to see how much fuel you spent. If I misread, sorry. If this is roughly what you did, then I am sorry to say I am not particularly impressed with the methodology (I realize this is the only thing you may be able to do - no offence meant at all - it is better than nothing). It cannot possibly be close in precision or reliability to direct observation of km/l or l/km. I hope the above gives you a good idea how I know. This is the least "theoretical" approach mentioned so far. All my "occasional observations" disclaimers mean that I didn't obsessively do it over dozens of trips, write down the numbers, run F-tests or whatever... > one thing to note - the car uses more then just fuel to recharge the > battery. every time i leave the accelerator (e.g. when coming to a > traffic light, or due to getting too close to a car in front of me) - > the battery is being recharged. I made a note of it in my very first post. -- Oleg Goldshmidt | p...@goldshmidt.org ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
Re: OT: Hybrid cars
On 09/17/2013 09:07 AM, Oleg Goldshmidt wrote: guy keren writes: "watching the numbers occasionally" is not a proper experiment. you need to reset the computer before you start the "drive under test", and check the value after - and the length should be enough to even out the fluctuations. I don't know what your car shows you. Mine directly shows the fuel consumption at the moment (that jumps around) and a running average over some period of time that is updated every few seconds. I don't remember what the averaging period is exactly, but if you drive at a steady speed for a while (I have cruise control, too) you will get a pretty stable number on the screen. watching the "current consumption" numbers can be quite missleading, since' during a lengthy period of drive, the number is usually not stable, and the assumed summing up of the numbers isn't necessarily the real summing up of the numbers. These are two different screens on the dashboard that I can switch between with a button on the steering wheel. The computer screen is right next to the speedometer, so I can watch the speed (even without cruise control) and fuel consumption simultaneously. In principle, I think there is another screen that reports your running average speed, but I don't think I used it for this purpose. I don't know what you mean by "resetting" the computer. I assume you reset the "trip distance" counter. I don't even need it to watch the fuel consumption numbers. one of the fuel consumption parameters the Prius gives (and it also existed in the renault megan i had on year 2000) is the "average fuel consumption since the last reset" - and you can manually reset this counter whenever you want - so it allows you to reset the counter, perform a drive of any distance you wish (1km or a million km - doesn't manner) - and get the actually (computed, not guesstimated) fuel consumption you had across the entire drive. to me - this is the *only* number that counts, since the other numbers are not steady enough across a long drive. It sounds like you took a trip with a full tank, guesstimated your average velocity, and topped the tank again to see how much fuel you spent. If I misread, sorry. If this is roughly what you did, then I am sorry to say I am not particularly impressed with the methodology (I realize this is the only thing you may be able to do - no offence meant at all - it is better than nothing). It cannot possibly be close in precision or reliability to direct observation of km/l or l/km. as i said - i let the car's computer (together with the resetting i mentioned). since i also have cruise control in the car, i can also assure a fixed speed across a given distance. and by the way, this fixed speed does NOT generate fixed fuel consumption across a long drive. very tiny changes in the road's slope (even on what superficially appear to be a flat road) bring it up and down quite dramatically. in fact, driving without cruise control and adjusting the speed to the changing road conditions allows you better fuel consumption then using cruise control. since the Prius's speed meter is digital rather then analog, you can see exactly how a change of speed of even 1km/h sometimes has a dramatic effect on the fuel consumption. even more - sometimes keeping the same speed but slightly changing the pressure level on the accelerator - can change the fuel consumption considerably. I hope the above gives you a good idea how I know. This is the least "theoretical" approach mentioned so far. All my "occasional observations" disclaimers mean that I didn't obsessively do it over dozens of trips, write down the numbers, run F-tests or whatever... my observations showed me that what i guessed to be the fuel consumption based on watching the "current consumption number", and what i actually used across a 5 minutes period, or across a distance, can be completely different number, and thus the former is not a useable measure if you want to know how much fuel you've eventually used across a distance. --guy ___ Linux-il mailing list Linux-il@cs.huji.ac.il http://mailman.cs.huji.ac.il/mailman/listinfo/linux-il
