As surprising as it may seem to the hardcore fuel economy nuts who frequent Ecomodder, there are people who have never given much thought to the process of "manually" (ie. without benefit of a ScanGauge or other onboard device) figuring out a vehicle's fuel economy.
I had someone ask me how to do it yesterday.
After the initial shock wore off
I came up with these instructions. Feel free to comment/add suggestions.
1) Next time you get gas, *fill* the tank. You can't calculate mileage if you're in the habit of just adding $10 or $20 worth each time. So fill until the pump clicks off once or twice.
2) Reset your trip odometer - if you have one. If you don't have one, write down your current odometer reading.
3) Drive, drive, drive...
4) Next time you need fuel: again, completely fill the tank until the pump stops. Get a receipt.
5) Write on the receipt the distance from your trip odo (or regular odometer) you went since the previous fill up.
6) Divide the distance driven by the amount of fuel used. That's your fuel economy. If your odometer is in miles, and the receipt shows gallons, the resulting number is your MPG (miles per gallon).
E.G. let's say you went 360 miles and you completely refilled the tank by adding 8.7 gallons (US).
360 miles / 8.7 gallons = 41.4 MPG.
7) Keep doing this each time you get gas. Save your receipts/calculations in your glove box or wherever. After a few fill-ups you'll be able to see what's "normal" for your car (assuming you do the same type of driving for each tank), and you can start using this info as a baseline from which you can try to improve.