MPG Calculator

MPG = miles driven / gallons used. To measure it honestly, fill the tank all the way, reset the trip meter, drive normally, then fill all the way again. The gallons on the second receipt are exactly what the trip used. Using the first receipt instead is the most common mistake, and it gives you the MPG of the trip before. Cost per mile is the gas price divided by MPG, which is why the money figure and the MPG figure move in opposite directions. The metric conversions: L/100 km = 235.215 / MPG, and km per liter = MPG x 0.425. The first is the one used in Europe and Canada, and lower is better, which trips up anybody used to MPG.

How do I calculate MPG?

Divide the miles driven by the gallons used to refill. 312 miles on 11.4 gallons is 27.4 MPG. Use the pump reading from the fill-up at the end of the trip, not the one at the start.

Why is my real MPG lower than the sticker?

EPA figures come from a standard test cycle. Short trips where the engine never warms up, cold weather, roof racks, low tire pressure, heavy loads, idling and hard acceleration all cost several MPG. Highway speed matters more than people expect, because drag rises with the square of speed.

Is the MPG readout on my dashboard accurate?

It is usually optimistic by a few percent. It estimates fuel use from the injectors rather than measuring it. The tank-to-tank method used here is the honest check, especially averaged over three or four tanks.

What is a good MPG?

For a gas car, above 30 is good and above 40 is excellent. The US new car average sits in the high twenties to low thirties. A full size pickup in the high teens is normal for its class.

How do I convert MPG to L/100 km?

Divide 235.215 by the MPG. 30 MPG is 7.84 L/100 km. Remember the direction flips: a bigger MPG is better, a smaller L/100 km is better.

Does driving slower save gas?

Up to a point. Most cars are at their best somewhere between 45 and 60 mph. Above that, air resistance rises steeply, and every 5 mph over about 60 costs roughly the same as paying an extra 20 to 30 cents a gallon.