Warnning: MPG gauge meter is inaccurate for all prius

I have checked mine and it is off less than 1 mpg for all the miles on the car. that is close. I agree that it should be set to give some over and some under if it has deviation. and I agree that getting a little better mpg on the display makes many people happy with their assumed mpg.

If you want to be shocked by what the gov. allows. how about many cars that do not get near the epa esitmate mpg. that would be much more upsetting. buy a ford hybrid and get 40 instead of 47. one may buy this car thinking they will get almost the same as a prius then get 10 mpg less. that would frustrate me. and they just keep listing the epa mpg like it is accurate.

 

Since purchasing my 2013 Prius C last July, my actual fuel mileage is 55.3 mpg over 3283 miles at the most recent fill-up. The indicated mileage is 57.9 mpg on the MFD. A difference of only 2.6 mpg, so the MFD is 4.6% higher than actual. I think it's very satisfactory accuracy. And the car is great.

 

I think it is the volatility of gasoline that makes it difficult to display mpg accurately.

There are 2 paths that the fuel is output from the fuel tank. One is through the fuel injector in liquid form and the other is through EVAP system in vapor form.

Being a liquid, it is not difficult for the ECM to compensate for the measurements of fuel injector pulse width due to variations in ambient temperature, intake air temperature, atmospheric pressure, battery voltage, fuel pressure, intake manifold pressure, temperature of fuel, etc. ECM uses this pulse width to calculate how much fuel has been consumed.

OTOH, determining the amount of vapor is much more difficult. Here is a link to evaporative emission control system on how it works. The EVAP system is designed to store and dispose of fuel vapors normally created in the fuel system; thereby preventing its escape to the atmosphere. It delivers those vapors to the intake manifold to be burned with the normal air/fuel mixture.

IMO, these vapors are not taken into account during the fuel consumption calculation, the ECM reports less fuel consumed and hence higher mpg is displayed.

Vincent

 

It is, in the real world, not possible to manufacture an accurate mileage system that is cost efficient. Assume there are four measurements needed to calculate the mileage, and they are each 99% accurate (a very high effciency). The total efficiency is then about 96% (.99 X .99 X .99 X.99). This four percent can be plus or minus. Getting that last four percent is a job for NASA, not Toyota. The same efficiency chain applies to the gas pump and the calculated mileage. If one is high and one is low, the difference can be substantial if you are looking for real numbers. I have always used the information from the car as a guide but not an absolute number. If your mileage is going up, you are doing something right. An OBD device has the same efficiency issues, and very often my ScanGauge will show a higher segment mileage than the car.

 

With our 3rd gen hatchback, Toyota has managed amazingly consistently inaccurate readings:



My take: they've pushed it just as far as they can, without it seeming ludicrous.

 

To the OP. Just go one size bigger the next time you replace your tires and the MPG meter will give lower readings. Of course your ODO and Speedo will also give slightly lower readings (by the same percentage) so be careful not to speed.

JeffD

 

How is this impacting your life? You figured out the meter may not be accurate, but is not that much off, so how has this impacted you?

 

I use Fuelly - have from the first tank as I had already read about the MPG "issue" before I bought the car.

The difference is around 3 MPG - but I'm quite happy with what the car is getting so I don't fret over the discrepancy.

Note the Prizm mileage - don't think I have a lot to complain about regarding the Prius.

YMMV

 

Yes, there was a case where Honda overstated the mpg on its Civic Hybrid. They overstated it on the sale's window sticker. It said that the average mpg was around 43. However, most owners experienced much less. I personally owned a 2002 and the most I got was 38 mpg. This was only achieved by driving like a turtle. I was pissed! I would have bought the Civic EX if I had know this because it came with many more options like a sunroof. I did receive a check from Honda.

This case was not about overstating the displayed on board calculated mpg.

 

Everything changes the manual calculation. How much fuel did you put in the tank(do you know to the exact ounce)? What PSI do you have your tires set at(this will change the odometer reading)? Did you record your mileage based on the tenths of a mile (or did you do to the rounded mile)? Each of these will change your readings slightly and will eventually add up. The only way that is closely accurate is to run your tires at exact manufacturer specifications and top the tank off until you see fuel in the fill-up (not recommended as it goes against Toyota's recommendations).

 

Maybe the 5% is at the gas pump. Maybe the car is dead accurate, and the petrol stations are telling you you're getting 5% more fuel than they're actually dispensing. The dash is correct; you're calculations are wrong... LOL

Such anguish over a toy on the dash display. That's what it is, you know: A toy.

 

I would like to see + or - not always over stateing mpg. I am ok with it but it should not always be less than the computer says. I use these numbers to try to figure out how many more miles can I go. because the car tells me to fill up with almost 100 miles left in the tank. I know from the past that my mpg is about 1 mpg less than it shows.

 

The gauge to me is just a ,,,, about ,,,,,, just fill up ,,, say 9 gallons 360 miles
You got 40 mpg .... Which brings up another thing ,,, one can not purchase
A gallon of gas , at least around here.... As the price per gallon is 367.9 per
Gallon can't find a 9/10th cent

 

I'll trade - filled up yesterday. It was $4.13 9/10 per gallon.

Late last night - the price had gone up...

Should have filled up both cars early yesterday.

YMMV

 

Is there a way to adjust the displayed mpg? It needs to be multiplied by 0.95 to get the real mpg.

 

Short answer: NO.
And anyway the error probably isn't linear in all situations.
It is just an "ESTIMATE".

 

Toyota should adjust the mpg display and show 1 mpg less on my car. Many others have a larger difference, however they don't adjust them. I guess many people are excited thinking they are getting a little better mpg. IMO there is no reason that they cant adjust so the mpg is closer to actual. I know it will not be perfect but this gauge will never under estimate the mpg. only over estimate the mpg? In my car the lifetime mpg is less than 1 mpg off so this is a small thing, but it does iritate me a little.

 

Umm, no.

The minor variations introduced by variations in fillup, tire condition, and on and on: cancel out, with as few as two fill ups.

If you fill up 'till the pump shuts off automatically, record your odo reading, do the math, and repeat: 2 or 3 fillups down the road you'll have a near deadly accurate estimate of your mpg.

 

Really? I wasn't aware that scientific process considered two inaccurate readings to become an accurate one...

You say that if I fill it until the pump shuts off it will be the same every time. I've used the same pump twice in a row and had it shut off in two different spots of my tank before. This is where the variation comes from. Different pumps do things differently... shoot, the same pump, a week apart, can do things differently.

 

If you keep track of your mpg with several fills, and lots of miles then your readings will be more accurate. I have kept track of mine on fuelly. all you have to do is fill up, write the odometer reading on the reciept. enter the info on fuelly. this only takes a short time. the fuelly site will calculate your mpg.

you posted that odometer readings are wrong unless tires are inflated just right. I have not done any testing of this, I think I will at least double check my odometer with a gps