Determine the total number of miles driven and your car's average miles per gallon. We'll use the example of a person who drove 500 miles in a car that averages 40 miles to the gallon.
Divide the total number of miles driven by your car's average miles per gallon. In the example, 500 miles divided by 40 miles to the gallon equals 12.5 gallons of gas. This is an estimate of the gallons of gas the person used.
Divide the total cost of the gas you bought by the number of gallons you drove. In the example, let's say the person paid $37.50 for gas. Therefore, $37.50 divided by 12.5 gallons equals an average gas price of $3 per gallon.
