Warnning: MPG gauge meter is inaccurate for all prius

It's not that simple as others on this thread have demonstrated that their error is much less than 5%. Who knows, it could be that gas pumps always err on the low side (as much as they can legally) so none of us are putting as much gas in the tank as indicated and then the meter on the car is measuring actual gallons burned so some of the difference is due to gas pumps.

 

The error seems to vary by Prius model, with the newer c and v Toyota has apparently reduced the exaggeration percentage, a step in the right direction.

 

Thanks

I had one tank that the mpg was off like 3 mpg. I got over 70 mpg on that tank, so I think it may be related to trying to get crazy mpg. The weather was warmer it was last fall. So I expect as weather warms up I may see a larger overstatement of mpg. Other than that the mpg seems really close usually about .75 mpg

I am not upset that my mpg is inaccurate, what bothers me is that it always overstates the mpg.

 

I noticed something I never noticed. I usually take a quick glance at the MPG as soon as I arrive any where. But this time after I parked the car and waited in the car to get my things together, the MPG went up from 51.1 to 53.2 after about 60 seconds of being in the car. I think some of the inaccuracy is when we usually park the vehicle and turn it off with in 5-15 seconds of arriving. We do not let the computer properly calculate MPG. Try it out, I tried this multiple times today, and the MPG went up, as well as down after I parked. I made sure that the ICE did not turn on when I parked. I made sure that I had enough juice to stay in EV mode while parked. So I will keep track of the real world MPG on this tank of fuel and the next to see if the inaccuracy is lower. Anyone one else willing to spare a minute (literally) and try and see if anyone else's MPG changes after being parked?

 

Could the difference in MPG be due to the ethanol that is added to fuel nowadays?

 

The complaint is not about the mpg, it is that the MFD always says you got better mpg than you really got. many people get like 4 to 5 mpg less than the car says. and this seems across the whole automotive industry not just prius. any car I have had always says you are getting better mpg than you are. automakers should adjust mpg or at least set these computers up so the owner could adjust.

 

With our previous car, a second gen Civic Hybrid, the displayed mpg was always a little low, ie: it underestimated. Just slightly. And very consistently. If for example it reported 5.0 liters/100 km, when I calculated I'd either get the same or 4.9 (the latter being better in the metric method).

It's likely running against the pack though, it's just too tempting for the manufacturers, to pad the numbers.

 

This is Bill speaking. From an engineering stand point, all Gages have production tolerances. You can buy lab quality Gage's with +\- 0.1% accuracy, however you will pay mucho dollars for them and you will need to have them recalibrated once a year or so to know the correction needed. Does it seem even remotely possible that the low dollar C that we own came with lab quality Gage's? Gasoline varies as much as 4% density in any given load from the same supplier. So even an accurate, expensive gage will appear as inaccurate when it is due to fuel changes beyond it's control even if you burn ONLY the 87 recommended for the ICE. By the way do not burn higher octane gasoline as your mpg will actually go down due to delayed fuel burn which sends fuel out the exhaust system and lowers the life of the cat. Converter; it is like paying for it twice! Further, we have owned 36 different cars over a period of 55 years. All speedometers, odometers and mpg indicators were off by as much as 8% except the speedometer on a 2005 Mercedes E320 CDI which was exact when measured by stop watches and later with gps. Suggestion: fill your tank at the same station, same pump and headed the same direction till the first pre set detent just clicks off the first time only and reset your trip odometer to zero and note the outside air temp and whether it is raining or not. Repeat this when you have used at least 3/4 of a tank. Divide the miles traveled by the fuel used for actual, as close as you personally will ever get it, mpg. Your car will get lower milage when driven in the rain, or uphill, into a headwind, crosswind, low outside air temperature, low tire pressures, a dirty or non waxed exterior, high relative humidity, and on and on. Soooo many variables!

 

I think the accuracy of the gauge is fine. It is the calibration that always says you are getting more mpg than you are. I have 11 fuel ups and every one the display over states the mpg. this is not an tollerence issue. I always fill up at same station same direction and it is only filled to first click.

As for using fuel with higher octane, I dont believe there is a reccomendation by toyota, my handbook says 87 or greater octane. my mpg with 92 octane e10 seems to give 10-15% better mpg on my car and driving. I am doing a longer term test where I fill up with 87 e10 and run tank record mpg and fill with 92 e10 and do the same. I have used 2 tanks of each and 92 seems much better, my last tank I ran for 600 miles. any way I will know for sure if it is better within a year of doing this test.

 

Calibration for an instrument is when you remove the gage, send it to a certified lab who in turn puts it on a certified test bench and test it against KNOW ASTM standards. There is no way to know it is fine or not unless those steps are taken. How accurate are gas pump Gages? Was each tank run in identical conditions? That is why it is best, as you say, to average annually, yes for the entire year and even over the entire life of ownership. For example, the Mercedes diesel averaged 36.5 mpg over 117K miles and 9 yrs having been in 20 states of the 50 (It was rated 27 town and 37 highway). Thanks for your interest in reading this.

 

You have thrown in many variables, just in this statement, that can cause that variance. But people are still angry that it's wrong. Maybe Toyota should just take it off the car altogether?

 

Nope, that isn't it.

The vehicle's weight flattens the 'round' tire where it contacts the road, so the familiar circle equations fall apart. We have found that with modern steel belted radial tires, the effective rolling circumference is closely tied to the length of the belts under the tread. This changes much less than you'd expect from the normal circle equations, even when the car is noticeably raised by increased air pressure. Any change here is a small fraction of a percent, not the 5% you are looking for.

In regular nonhybrids, which in the U.S. are normally geared low for performance instead of high for economy, increasing tire diameter/circumference will improve actual mpg. The Otto-cycle engine under normal cruise conditions is quite inefficient, and the taller tires shift the effective gear ratio in the direction of better economy. But you are right that the odometer doesn't correctly record this improvement because it knows only wheel rotations, not actual road distance.

But the Prius's Atkinson-cycle engine and HSD have already applied other remedies to reclaim the above inefficiency. There is little or nothing left for a different tire size to reclaim.

The GenIII MPG display is simply wrong, with a built-in optimistic bias.

 

So is the speedometer.

 

... for completely different reasons.

The GenII Prius, and several other car makers, proved that MPG displays can be accurate.

 

This newyorker guy seems like a tool ish noob. sucks man. If it makes anyone feel any better the miles driven vs odometer with stock 15" steel wheels and 175's is different. more miles are travelled than the odo records so it seems as though the ecu is programmed for the shorter 16" 195 tires.

 

With only a handful of fill-ups, the difference between my calculated MPG and that reported by the MFD has ranged from -1.28% to 9.15%. The mean is 3.01% and the median is 3.14%. This is not a big deal to me. In fact, calculating the ongoing difference between the two is kinda fun. If the difference were 10% or more, I'd be annoyed (but with all other things being equal, I would still choose to own a Prius).

 

Made sense to me. I just don't know how accurate it is. He's saying that he thinks the car is spec'd for the 16" wheels and not the 15" wheels. That will change the odometer reading which in turn will mess with the MPGs.