Fuel mileage measurement

Why do the automatic fuel mileage measurements on both my 2006 4Runner and my wife’s 2006 Volvo XC-90 average about 1.5 MPG more than my calculations using actual gallons and actual mileage?

Your fuel mileage “meter” is a computer program that simulates your driving. I had a Ford Explorer for some time on a project and it did the same thing. Do not attach to much value to those readings; they are best used to compare driving speeds with respect to milleage. Actual measurements from the same gas station, same pump, will yield what you actually used over several tankfuls.

Because they’re NOT accurate. I ignore mine completely.

The computers use averages based on recent data input.
Accelerating onto the highway can give recent data that changes the display for the next few reads. ( based on CURRENT info your average mpg would be X, says the computer )

After driving around town all week my display will show X miles to empty, then I head out on the freeway to Albuquerque and get much better that that ( hiway mpg being better that in-town mpg and the computer can’t know what I’m ABOUT to do next. ).

It is safe to say that no meter, no matter how expensive is perfectly accurate. Not your gas gauge, your oil pressure gauge, your tach, your speedometer, your odometer, your tire pressure gauge and more.

I can imagine that the gas mileage calculation is a simple process for the computer. Inputs are odometer and fuel injection time at a known fuel pressure to establish a known fuel flow rate. Possible gross sources of error are the odometer and fuel pressure. The fuel pressure regulator may not be exactly at the pressure that the computer assumes. I wonder what is done when the vehicle is at idle when the mpg reads zero. In reality the mpg is less than zero when the vehicle is at a standstill with the motor running and I wonder what the mpg computer is doing with that.

Gas station pump meters are also not perfect. I was told the error tolerance permitted by a pump maintenance person but can’t recall the limits but do recall that the error tolerance according to the numbers given to me tended to be in favor of the customer.

The temperature of the fuel that you bought compared to the temperature that you used it at is another source of error. That was and possibly is still an area of controversy. You get more for your money if you buy cool or even cold gasoline. I heard a story a couple of years ago about a gas station in a warm climate that ran its gasoline through overhead pipes to warm it before it went through the pump meter.

Nonsense. Gasoline barely expands before reaching the evaporation point. The volume is practically equal at 0 degrees as to what it is at 90 degrees.
Absolute compelte and total rubbish.

It’s really very simple…The guy who programs the mileage computer function is SURE to set it up on the optimistic side… Car makers want to look good, not bad…After all, the computer knows EXACTLY how much fuel was injected and how many miles were driven…