[BHarvey]

If you only buy $20 at a time, as gas prices rise you'll be getting less fuel which would show a false improvement.

Example- 500 mile odo reading.
$1/gallon = 20 gallons= 25 mpg
$2/gallon = 10 gallons= 50 mpg
$3/gallon = 6.66 gallons= 75 mpg