MPG, which stands for miles per gallon, and GPM, which stands for gallons per mile, are two ways to measure the efficiency of a car. MPG has been the standard measuring stick for cars' fuel economies, but some argue that GPM is a better measurement. With GPM, you can see exactly how many gallons you use when you travel a certain distance, which can make a cost comparison easier. For example, looking at a 13 mpg car versus a 15 mpg car may not seem like as big a difference as a 30 mpg car versus a 40 mpg car, but over 1,000 miles, the 2 mpg difference between the 13 mpg and 15 mpg car actually results in a larger savings than the 10 mpg different between the 30 mpg and 40 mpg car.
Determine the miles per gallon of your car, truck or other vehicle. This is usually listed in the vehicle's owner's manual.
Take the reciprocal of the miles per gallon measurement. The reciprocal is found by flipping the numerator and denominator. For example, if you had 25 mpg, which is the same as 25/1, the reciprocal would be 1/25.
Use a calculator to convert the fraction from Step 2 into a decimal to find how many gallons it takes your vehicle to travel one mile. For example, 1/25 equals 0.04 so if your vehicle got 25 miles per gallon, it would convert to 0.04 gallons per mile.
