Join Date: Sep 2007
Location: Louisville, KY
Car Year: 2008
Car Model: LL Bean (4EAT)
One thing that you can do is eliminate the variables that result from topping off. Only do fill ups after some significant consumption or distance, like 10 gallons or 200 miles, and add no more after the pump shuts itself off.
The calculations on a top-off can vary greatly and do not give a true view of mileage. The few gallons added in a top-up are influenced little variations in how far the fuel splashed up in the filler neck to shut the pump off, or whether you added another click of fuel. Topping off can result in an under fill or an over fill.
However, the variations due to topping-off do cancel or average out. If you under fill the tank at one fill up, the tank will require extra gas at the next fill up. The good mileage of the first fill up will be balanced by poorer mileage of the second fill up.
Although your mileage may appear to be varying wildly on individual fill ups and top offs, the overall average of a half-dozen tanks will be more accurate and acceptable.