ahh the fatal flaws...

1. you believed the receipt from the pump, you dont KNOW how accurate this is, also unless you ru nteh car till it stops on the flat, then chuck in a gallon and repeat its almost impossible to do accurate short term calculations (hint gas punps dont stop at exactly the same point at a 'full' tank)

2. if you think abotu it the way the display must work is as follows:-

milage driven (taken from electrical pick up on the road wheels/ road wheels side of the transmission), fuel used taken as a mathmatical calcualtion accoring to the ECU's record of durration of injector pulse (i.e the amoutn of fuel given to the engine at any time can be derived fro mteh length of injector pulse the ecu gives it - probably caclualted on fuel mass as it happens).

now the injector pulse stuff is likely to be very accurate - given i bet the code in the ecu work out the mass of fuel it wants THEN calculates the injector pulse length...

the revolutions of the road wheels is accurate in rpm terms, but becomes an issue when converting to distance terms...

lets consider 16" wheel wearing 205/50r16's...

tyre diameter = 636.8mm

so circumferance PI*D = 2014.8352mm

but lets assume thats a NEW tyre, now we can easily take 10mm wear off a tyre when its old...so we now get

tyre diameter = 1969.4056

or about a 2% inaccuracy...

aint life grand...

so the ONLY way i would assume the trip computer average to be accurate is over an extended period, but after all thats what averages are all about...