Calculate the number of miles per gallon of fuel.

The easiest way to calculate how much fuel you use travelling a certain distance is to fill up your tank write down the miles from your odometer. Next, fill up, look again at your odometer and subtract the first odometer reading from the second one. This calculation will give you the distance travelled by car between fill-ups. Divide this amount by the number of gallons it took to fill up your tank at the second fill-up to get the miles per gallon amount.

distance ÷ fuel = miles per gallon

Example Calculation

Fill up your tank and look at your car's odometer.
First odometer reading =  34678.7 mi

The second fill-up can be any time after the starting one, but if you are trying to figure out how many miles per gallon, wait with your next fill-up till your tank is closer to being empty for a more accurate result.

At your next fill-up, your odometer reading is 35026.48 mi, and it took 16 gallons to fill up your tank.

Subtract the first reading from the second to get the distance;

35026.48 - 34678.7 = 347.78 

Between fill-ups, you travelled a distance of 347.78 miles

Divide the distance by the amount of gas it took to fill up the second time;

347.78 ÷ 16 =  21.7 

You get an average of 21.7 miles per one gallon of fuel.