You don't have to use a whole tank to get the mpg, but it provides a better average.
Just fill the car up, reset the odometer trip and drive. When you've used about half a tank, go fill it up again, note the gallons the pump puts into your tank and divide that by what the trip says. It's that easy.
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'13 Mustang 3.7V6 - MT82
'00 GP SE (L67 Swap, XP, Headers, 3.25")
'98 GTP, '01 Bonne