# Discrepancy of MPG between on board computer and at the pump

I am seeing a discrepancy between the readouts, when I calculate MPG in my 2007 Toyota Camry Hybrid at the pump. The read out on the console is usually anywhere from 3 to 5 mpg better than what my calculation is after filling up at the pump. When I asked the dealer to check to see if the on board calculator was reporting accurately, she said the on board one was working fine. She said it is just calculated differently than when at the pumps. I have no idea what that means. How can both be correct? How would they check the accuracy of the on board calculator? Thanks for any insight you may have.

The pump wins. On board computers are known for lack of accuracy. It may allow for some adjustment to get it closer to correct. You also may want to check your odometer and speedometer. If you have a good GPS they are usually very accurate by comparison.

Generally they are good for proving to you that you are getting a lot better mileage at 55 mph than 75.

The car MPG calculation is NOT accurate. I’ve yet to see one that is even close.

The MPG readout on my 2006 Sienna is consistently ~0.5 mpg higher than the calculated MPG (miles/gallons). I’m surprised the numbers are that close.

However, I calculate the MPG over 3 or 4 tanks of gas (900-1100 miles). I reset the Sienna MPG readout every 3 or 4 tanks also. The MPG readout is a running average from the time it was reset no matter how many miles are driven. Try this method and see how close the MPG numbers are.

Ed B.

The onboard is not calculating this tankfull, you are. the onboard is running a grand total average. this is the “difference” she speaks of. For the most accurate picture keep your pencil and your math skills sharp.

A mechanic can only check to see if the computer is functioning normally, but he has no way to check the mpg calculator for accuracy. Continue to use its numbers as a crude guide or to give you bragging ammunition. Only your math skills can determine the true fuel economy.

Most “MPG” gauges are trying to accurately measure instantaneous distance traveled AND fuel flow rate. Even being off a tiny amount in either can throw the MPG number way off. Gauges that try to report distance and fuel consumption integrated over some period of time (a moving average) will usually come closer to the mark, but nothing can beat an odometer reading and gas pump number. Use an MPG gauge only for comparing different steady-state regimes such as cruising at 75 vs 55, etc.

The on-board computer is just a toy. It is not intended to be very accurate. It can’t predict future driving conditions either.

I’m always a half gallon off or so from what the computer says. No big deal. Knowing whether you get 38 or 40 MPG won’t change the actual facts at all. I usually just look at the total dollars and calculate cost per mile over a period of time instead.

The onboard computer display will at least be more consistent. There is a lot larger margin for error with filling the tank.

The MPG indicator in my Lincoln is very accurate and matches the pump reading.
As I’ve mentioned on this forum before, it goes off on a tangent about once a year and gets a bit goofy for a few days but 99% of the time it’s near dead on.

There was a disussion recently about MPG as compared to speed and a few were somewhat derogatory towards me when I stated that some vehicles, NOT all, may get better economy at higher speeds than lower ones. It all depends on a number of factors.

Since I have verified this on a number of road trips previously and was about to take an out of state trip to CO I stated that I would gladly perform this drill again and keep an accurate log of both the dashboard display and the pump/odometer readings.

Since I don’t want to distract from this thread too much, I’ll dig the notebook out of the glovebox tonight, do the math, and post the results on a separate thread this evening.

what milage would you like?

tell us that and we can change it with the scan tool,neat huh!

its a guess,thats all,and a bad feature.

Our 05 GM car’s computer read about 1 mpg high. Our current GM car, an 08, read about 1 mpg high and 1.5 mpg high; checked it two times so far.

## The displayed MPG comes from the cars computer, which is extremely accurate, as it must be to pass all EPA requirements. What causes discrepancies is the a combination of factors such as idle time, gross inaccuracies in manually measuring tank fills, etc. I deal in diesel engines and so am not really competent to discuss gas but here is a response I copied once from a gas knowledgeable person. I didn’t keep his name so can’t properly attribute to him.

The only way to compute fuel consumption on an EFI engine, and the
way the OEMs do it is to totalize the injector open time and,
knowing the injector dynamic characteristics, compute total flow.
It is not as simple as just knowing the flow characteristics of the
injectors. One must also know the dynamics of the injector since
under normal conditions, the injector spends so much time opening
and closing. A typical injector can open in a millisecond. It
takes a similar amount of time to close. During the opening
process, the flow goes from zero to full flow in a very non-linear
fashion. High speed photographic studies I’ve done on the old style
Pintle injector (where the valve pintle is visible) shows that the
flow doesn’t start until about 750 microseconds into the open
event. The closing process is also nonlinear but of a different
shape. During idle and cruise, the injector is running with an open
period of from about 0.5 ms to perhaps 1.5 ms. Under these
conditions, the injector is continuously either opening or closing,
never reaching the full open state (or just barely doing so.) The
OEM has characterized the injector and driver and knows the transfer
function. The OEM also knows the characteristics of his fuel rail
pressure regulator, another vital input parameter. Without
specialized test equipment, you can’t.

If one thinks that dashboard fuel displays are inaccurate then see my post about MPG vs Speed.

The variations are extremely slight (under a tenth), The onboard display narrows it down to a tenth of MPG so I would consider mine at least very accurate. As I’ve mentioned in the past, it takes a spell for a couple of days each year when it develops a case of the stupids but after that hiccup it’s fine; and accurate.

as I said,a B/S,wild guess.

My '05 Golf computer always comes out about 1.5 MPG above actual (about 5% off). My speedometer is also about 7 percent off (reads fast) - probably due to different than original tire size.

So overall, the really aren’t off at all to any degree. Considering it is not measuring the amount of fuel used, but instead the amount of fuel requested to be used by the fuel injectors, it’s rather amazing it’s that close.