The simple/easy/accurate way to track to track your total mpg: save and record all the gas receipts. All the gas put in, divided into the miles driven (the odometer), that's your total mpg. Didn't start that eight-o'clock-day-one? Never to late: for every fillup, note the gas added and the odometer reading, that's all you need to sort out the calculation. Work it in a spreadsheet and you calculate individual tanks and overall.