Warnning: MPG gauge meter is inaccurate for all prius

Anyway, why is this even in the Prius C section or finger pointing at the Prius in general??? Since for the most part, a Prius C and it's bigger brothers meets their advertised MPG (unlike other companies out there)???

 

Most people can't calculate, so who cares if the car can't either, its not the end of the world, you are making this into something more then it is, but thanks for nothing.

 

Mine's only off by 0.2 l/100km, no big deal.

 

it's a minuscule bit optimistic.. omg.. really?

dude.. it's a computer.. it calculates using various things that aren't always accurate.. so alas... inaccurate reading.

 

I think Prius c and v are more accurate. 3rd gen hatch tho is terribly optimistic.

 

This is not completely a Prius problem. Are you filling the tank to the exact same point each time? Are you taking into account the difference in gasoline temperature changes? Is your odometer exactly accurate (mine is not)? All of these things will affect the calculated mileage.

 

Drysider, any variation due to fill up cancel out over multiple tanks. Even just two tanks.

(Drysider, sorry for typo, can thank my Iphone's auto-correct)

 

Have you yet found any legal leverage to regulate the accuracy of these meters?

 

Is it just me, or does the article you linked seem to indicate the Prius is probably better than most in this regard?

I believe its pretty well established that calculating your mpg manually on a single tank is pretty inaccurate. This effect is exaggerated for high efficiency vehicles, since the same volumetric error on filling translates to more variation in calculated mpgs. Take for example a pump (or more commonly a variety of pumps) that has an accuracy of +/- 0.5 gallon with regards to its autostop, which seems reasonable as its purpose is not to accurately stop at a precise point but prevent overfilling. This 1 gallon difference in a vehicle with a 20 gallon tank and an 18mpg average economy translates to a +/- 0.5mpg error range in calculated mpg on a given tank. Now take the same 1 gallon difference in a vehicle with a 10 gallon tank and 50mpg average. The same volumetric accuracy now equates to a +/- 2.5mpg error in hand calculated mpg on a single tank, 5 times the mpg error of the less efficient vehicle.

The thread you keep mentioning shows several things. The accuracy of individual tank calculations is all over the place, the reproducibility of the computer mpg appears to be pretty stable. When you average the hand calculated data out over time it tends to be within 2-5% of the computer average.

Its interesting that you seem to be pointing out the extreme single tank errors as evidence that the computer is wildly optimistic, but ignoring the posts where the calculated mpg is 10-15% higher than the computer result thus evening out the averages.

 

All of this seems to be forgetting what the most beneficial result of putting mpg meters in cars is. Its not so you can plan your fuel budget as the article somewhat ridiculously suggests. As if a predictable 5% error in the gauge is going to break your budget in a market where fuel prices routinely swing +/- 10% or more over fairly short time periods. Its primary benefit is based in the research showing that people will generally drive considerably more efficiently in a vehicle with a gauge that gives them feedback on the efficiency of their driving than in one without. In other words its primarily about relative accuracy (teaching the driver what results in more or less efficient results in that vehicle) than it is about absolute accuracy.

It would seem like the most likely outcome of substantial negative public attention or lawsuits related to the accuracy of these gauges would simply be for automakers to stop putting them in. The system they use now is pretty cheap, as they just use the existing systems in place to monitor emissions. If it costs them more money to make the system more accurate, I doubt many of them would find that to be a worthwhile investment. There assumption is likely to be that customers don't want to pay more for a vehicle that has a more accurate gauge. Alternatively they'd be likely to move to non-numerical feedback, as some already have, which is even more useless to those desiring to gain insights into their absolute fuel usage/efficiency than a numerical gauge with a 5% error

 

maybe you should sue Toyota ...... how about trying to find a New York Attorney that will represent on such a great class action case - and keep us apprised of how that's working for you

 

The running measurement system requires the integration of large numbers of small elements. The measurement tolerance on small elements is always greater than on a single large element. If the running measurement was precise and was then manipulated by the application of a factor, the error would be constant, but it never is.

I have just refilled my PiP for the third time, the percentage error on the third fill (+5.7%) was much greater than on the second fill (+1.21%). If the errors resulted from a "fiddle factor", I would expect them to be pretty much the same as each other.

If I am asked whether I think a manufacturer is likely to design the system in such a way that the errors are likely to be in the "flattering" direction, then I would say of course they will.

If you want more accurate consumption figures use Fuelly or one of the other similar systems.

 

Let's not forget that the speedometer is required to NEVER read below the actual speed with a certain degree of error, which will always be on the high (i.e. conservative) side, typically around 2-3 miles. "If" the speedometer value is used anywhere in the mpg calculation, and I have no idea if it is, then of course the car-calculated mpg will read higher than actual. See Speedometer Accracy

But, as was mentioned numerous times in this (and other) threads, my calculated mpg will typically be lower than the car-calculated mpg by a margin of from 3% to 8%. Toss in all of the other variables which affect the efficiency of the car on a given day, in given weather, on a given road, while driven in a given manner, using a given brand of gasoline, maintained in a given manner, etc. - then I can certainly live with the variance, and I hope you can as well. If not, then, as mentioned, find a lawyer who will take the case and let us know how it all works out.

 

Just to blanket-comment on the various up-the-page technical reasons why Toyota can't make an accurate mpg display: they don't wash. And the obvious beneficiary from a rosy display?

Our previous honda civic hybrid had similar mpg display. It consistently erred, slightly, on the pessimistic side. That's about the only kind thing I can say about it, tho.

 

And a quick survey of the Greenhybrid forum shows typical error on the displayed mpg vs. time averaged calculation for an HCH is.... 2-3mpg (~5%) high.

 

My display always reads about 3 mpg more than actual calculations by hand after i fill up. So when i am driving around and i look at the display what ever it reads i just subtract about 3 mpg and it's about what the actual mpg is. Not a big deal really.

 

So when my display told me I went 500 miles on a single tank and got over 65 mpg, I really got only 60 mpg...

If 5% is the over-estimate, then I think that it would be fair for Toyota to adjust the software (e.g. during a maintenance appointment), so that if a customer asks to update the software that computes the fuel economy, Toyota should just be able to take the 5% into account and let the customer know if the software is changed.

(Lifelong I've relied on the computer to read off fuel economy... which has me wondering if, when I still had my SUV, if I was getting 17 mpg like it was saying, or if I was really getting 15 mpg...)

 

If, for example our second generation civic hybrid calc'd displayed mileage was 5.0 liters per 100 km, calculated would be 4.9 or 5.0. Without fail. Any other 2nd gen owners I've talked to reported similar.