Just for fun, let's assume the FE calculations done by the computer are based on the amount of fuel being used by the fuel injectors divided by the miles measured on the odometer. More specifically that 1) the odometer is correct (well within 1%), and 2) the fuel measurements are based on the commanded volume of fuel of each "squirt" of each fuel injector (4 cylinders @ 0.5 intake stroke per revolution is 2 squirts per engine revolution). For the sake of simplified calculation, let's calculate the squirt volume given for 40 MPG for 40 miles (1 gal @ 40 MPG) at 60 MPH
(1 Mile/Minute - which is 40 minutes), with the ICE turning 2500 RPM.
2500 Rev/Min * 40 Min/Gal * 2 Stroke/Rev =
200,000 intake squirts of gas for 1 gallon of gas.
That equates to about
5 millionth of a gallon per squirt, or
6 ten-thousandths of an ounce per squirt!
A bit more for lower MPG, and less for a higher RPM. This is probably a fairly typical volume of each squirt for my TCH.
The bottom line is that a very large number of extremely tiny amounts of fuel are being added by the computer to get the mileage calculation. I am quite impressed that it stays within 5% accuracy on my TCH!