The best way to check is to fill the tank to shut-off, reset trip odometer,
drive it, then fill the tank again to shut-off at the very same pump you filled
it at the first time.  Then do the math: miles driven divided by gallons
needed to fill it the second time.

Better yet, fill it and record the mileage.  Drive for a month, keeping track
of all the gas you put in.  Then fill it & check the mileage against all the
gas used.  That will give you a better average.

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL