Chris -
I would use a GPS in comparison to the car's odometer. It won't solve the 1.X % issue, but I think you will have the best possible accuracy. If you don't have a GPS, I'll bet one of your friends does.
In terms of inputing the percentage, you can always do a correction in your fuel log. That's what I do for my fuel log :
Code:
Fill date Distance Fill amount MPG
2008-04-07 194.7 mi. 4.946 US gal 39.37 US
Details
Ignore high/low MPG, only look at AVERAGE MPG (Chevron;87 Octane;$3.64;#9).
Tires are larger, so odometer is reading 2% less, hence +2%
correction: 190.9 * 1.02 = 194.718. EFIE off.
Needed to add 1/2 quart of motor oil during this tank.
CarloSW2