whats the advantage of documenting your gas mileage? if anything serious comes up, you'll notice it anyway. if i gets lower gas mileage, its probably cause i drove like a moron. i did it a couple times when the car was new to see what the car was about but now its worthless info to me.
if its budgeting your concerned about, then by all means do your documentation. but in the end, you have to drive your car and you know that slowing down will yield better gas mileage.
