If your gas mileage sucks, it will get 30 miles per gallon at 60 mph. That would be 1/2 gallon of gas in 1 Hour, 1 quart of gas in 1/2 hour, 1 pint of gas in 15 minutes, 1 cup (8 Fluid oz.) of gas in 7.5 minutes.
Point your gas hose into a cup, turn on the petcock, and time how fast it fills.
If it takes less than 7.5 minutes to fill, then it should be ok.
Cheers,