The only way to measure mileage accurately is to record the miles travelled between fillup's and record the gallons pumped. Average over several cycles since pumps cut-off at different levels. Verify your odometer against freeway mileage posts and/or a GPS. Some people actually think the tic marks on a dash fuel gage have some type of quantitative meaning, like the half mark means you have 1/2 the tank volume left. Never driven a car where that is even close to true.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans