This is not exactly true, unless you started with an empty tank, and even then you must add fuel to the exact same level each time, and use the fuel to the exact same level each time. if you run the tank more empty, it will skew your results. The current fill is equivelent to the amount of fuel used IF YOU STARTED WITH A FULL TANK. What you are actually measuring is the amount of fuel neede to replace the actual fuel used. This means that if you got your car with a half a tank of fuel, you must start you mileage recordings after you filled the first time, as that is your baseline measurement. You also must start your measurements with a full tank and end with a full tank. This does not mean that you can not add only 5 gallons here and ther, but you must get a full tank to get reasonably accurate measurements at the time you want your average fuel economy.

In other words, you are always using this fillup's gallons to determine the last fillup's fuel economy.

 

If you see my database the odometer at start was 0. Actually it was 2. Filled by dealer to full tank, which is 17.2 gallons. Over the various fill ups it does not matter when I filled it – half tank, quarter tank etc, as long as I record the amount of gallons filled. These get used by the car and the miles are added to the odometer. The final odometer divided by the gallons used (minus reserve in the tank) is the mpg of the car over its lifetime till that moment. If I want a true mpg now I must fill up when the cruising range comes to zero, record the odometer reading and divide it by the total gallons used prior to the fill up minus 3 gallons for the reserve capacity.


My point is that the variations will even out after 30 or 40 fill ups and at that point even recording odometer at a quarter tank fill up will not make much difference (maybe 0.25 mpg) to the mileage as only 6 gallons will be left in the tank. Obviously you cannot use the gallons of the very last fill up to calculate mpg, as I already indicated in my previous post.


I would welcome someone else who has the receipts to remove my readings and add their own for a few fill ups and see how their mpg is compared to the car’s interpretation.

 

Dont forget that the readings at the gas pump are not very accurate either. There is really no practical way of calculating accurate MPG without precision instruments.

 

Follow up questions: if the car's odometer is understating our mileage, doesn't the car's calculation of MPG use that understated mileage? If so this doesn't explain the discrepancy. Or are you saying that the MPG calculation uses some other measurement of mileage or some other way of calculating altogether? Also, does Toyota know their odometers understate the distance travelled? It seems this would be something they would want to fix...