I just use simple math. You can see in my Post #61 of this thread, I keep pretty good records on both MPG & Operating Costs (less insurance & depreciation). It really is just a matter of recording the miles driven on the Trip ODO whenever I fill up, resetting the Trip ODO to start a new cycle, recording the overall mileage on the Main OPO, & grabbing a receipt from the pump. That way I know Gas Price & Total Gallons to fill up. I record this in a Steno Notebook (6" x 9") that I have for each vehicle. Then I've got a MS-Excel file I use to keep track of both OP COSTS & MPG on an annual basis, plus any expenses I consider worth recording (the Steno Notebook has all expenses for the respective vehicle). I have this file for each of my vehicles. The file for my Crossfire is attached.