1. Determine the number of miles traveled. For example, suppose that you traveled a total distance of 208 miles.

2. Determine the amount of fuel used to travel that number of miles. Assume that the vehicle consumed 10.4 gallons of fuel to travel the 208 miles.

3. Divide the number of miles traveled by the number of gallons used to determine how many miles the vehicle traveled on 1 gallon of gas. Here, 208 miles divided by 10.4 is 20. Thus, the vehicle used 1 gallon to travel 20 miles.

Or, if you already know wht your MPG is. Go to mapquest, get your route, find the milage and work the mth from there to see hom many gallons you will require to make the trip.