Miles per gallon manual calculation vs car calculation

Deciding to trust but verify, I calculated miles per gallon by dividing miles driven since last fill-up divided by fill-up amount. I compared the resulting MPG with what the car calculation indicated on the dashboard. In the two times I did this, my MPG calculation is about 3 gallons per mile less than what the car calculation is. Which calculation would you believe? I do not want to be too suspicious but the manufacturer could easily program the computer to give me better mileage than actually is occurring.

In case manufacturer credibility is an issue, the car is a 2010 Prius and I am the original owner.

If you are confident that you did your calculations correctly, then–of course–you should trust your own numbers. However, you really should calculate your mileage on the basis of at least 3 tankfulls if you are going to get a reasonably realistic overall mpg average.

That being said, this topic has come up previously, and most of us have reported that our onboard mpg readout is very accurate. I have checked mine vs hand calculations on a number of occasions, and I have found the car’s calculation to be extremely accurate, albeit slightly optimistic.

If I recall correctly, my calculation resulted in an overall mpg figure of 24.5, and the readout on the dashboard reflected an overall average of 24.8, which is so close as to be…not an issue…as far as I am concerned.

Gallons/miles is right, as long as you keep track of it every tank to average out any errors. So with a Prius, you’re calculating, say, 40 mpg, and the computer’s telling you 43? Not that far off, to me.

Believe your own calculations, but your result can be in error if only two tankfuls are involved. Continue your record-keeping for at least five fill-ups and you will have an accurate result.

You should always believe your calculations, and it appears you did them correctly. But do you reset the mileage sensor when you refill the car? It will continue to average mileage from the last time you reset it.

But 4 of 4 cars with mileage sensors are accurate when compared to manual calculation. We have an Olds Silhouette, 2 Chevy Cavaliers, and a Chevy Cruze. Oh, I have a Honda Accord, too, but it doesn’t have this feature.

I have mpg indicators on three cars, an 06 Toyota and 2010 and 2013 Chevy’s. I calculate the mpg every 3 or 4 tanks of gas (1 to 1.2k miles). All three vehicles computed mpg is 0.5 to 1 mpg higher than my calculations. I’m surprised it’s that close.

Ed B.

I actually think the computer is more accurate unless you used the same pump, with the car at the same spot on the pad, and the gas has been temperature controled for expansion/contraction. You can keep track of it over several hundred or several thousand miles to be more precise but you will always have variations due to driving conditions, speeds, roads, temperature and so on. There really is no need for the precise calculation anyway since you just want to see if there are major changes over time to take corrective action. Unless of course you intend to sue Toyota.

I have a 2011 Prius. I always check the mileage. The car consistently says I am getting 58 mpg and when I check, my math always says I am getting 52.

The problem with the one tank method of calculating fuel mileage is you don’t always fill your car up the same way. Say you have a 10 gallon tank drive 100 miles then fill up, only the pump stops at 9.8 gallons instead of 10. Now instead of 10 mpg you’re getting 10.2 MPG or say it put in 10.2 gallons now your getting 9.8. Keep track over several tank then compare. I reset my average fuel mileage in my car drive for several tanks full then I calculate my mileage. When I do that they are so close that it doesn’t make any difference.

When reality and the digital alternate reality collide which wins? I once took a well known short cut through a rural community that was unknown to my passenger and the GPS and the image on the screen indicated that we were traveling over creeks, pastures and woods. The passenger was disturbed that I would drive where there was obviously no road.

After 75k miles on my 09 4runner, and having done the manual mileage calculation on roughly every second tank on my trusty sliderule, the manual mileage is always 1.5 mpg lower than what the car’s readout said. At the dealership, they say there is no way to adjust the calibration.

Your calculation is the correct one and seems you are doing it properly. I have had two cars with this feature. One was a Honda CRV and the computer on this one was consistently optimistic by 2-3 MPG. My current one is a Mazda CX-9 and after ~ 3K miles, the computer is right on the money to the 0.1 of MPG. I am ready to throw my pen away on this car.

I don’t think anyone should expect the digital readout to be accurate. If it is, it’s just a matter of luck. At least on my own cars the ECU has no measure of actual volumetric fuel flow rate, it must assume it from a calibration of injector “on” time and fuel pressure.

I’ve been calculating mileage manually on my 2012 Prius V for the year since purchased new (16K miles). The car’s computed mileage is consistently about 5% optimistic (including both manual and vehicle totals since new). I don’t buy that it’s just a coincidence, or that it can’t be adjusted. If it’s computed, the algorithm can be changed.

I like my car, but I was annoyed by the completely unhelpful response I got when I raised the issue with Toyota.

I don’t have a computer but the manual method is very constant. It doesn’t vary per tank very much. I get about 33 mpg in winter and 36 in summer.

"Your car is not counting time spent at stoplights or idling in traffic. "

?? The EFI keeps track of all the gas used, and the odometer keeps track of all the miles traveled, so it should have no trouble accounting for time spent idling in traffic. It just seems like some are not exactly calibrated. The errors I’ve seen reported seem to be “It’s always X% off”, which is a calibration error, not a measurement error.

I think it comes down to variations in calibration of:
MAF sensor, intake air temp, fuel pressure, injector spray rate, engine RPM.
I think those are all that’s needed (and not necessarily all five) to calculate fuel consumption.
I think 5% overall tolerance is pretty good.
That’s why the O2 sensor is needed to make the final tweak (fuel trim) from what is expected.

The fuel mileage indicators on my current and past Lincolns have been almost dead-on when comparing to the miles driven/gallons needed method.
It’s always within .1 at the outside most and to be honest, it surprises me quite a bit.

I’m not sure how my old '89 Grand Marquis measured the fuel used, but it was just about right on. It did take into account idling and so forth. used to have a bit of fun on a long steep hill kicking it into neutral and watching the computer figure 70 MPH at idle into 90 or so miles to the gallon. realistically Iwas getting 18 and that was what my computer averaged it all out to.

Like I said, how do you know how many miles you acutally traveled? Based on the odometer? How accurate is that? Spose it changes with the tire wear? I wouldn’t get too concerned with a 5% deviation and not sure what the purpose is for continual mileage calculation. I used to drive 40,000 miles a year and never tracked it except checking the computer from time to time. It would have cost me the same either way.