Gas Mileage Calculator

Your car's window sticker and dashboard display give estimates; the most reliable figure is the one you measure yourself. Fill the tank until the pump clicks off, write down the odometer reading or reset the trip meter, drive normally, and at the next fill-up note the reading and the gallons or liters it took to fill the tank again. Distance divided by fuel is your mileage for that tank.

One tank is a snapshot. Weather, traffic, a long highway trip or a week of short errands can move the number by 10% or more, so tracking several fill-ups and averaging them gives a truer picture. A sudden, lasting drop is worth investigating, because it can be an early sign of a dragging brake, a failing sensor or tires that need air.

In the United States fuel economy is quoted in miles per gallon, where higher is better. Most other countries use liters per 100 kilometers, where lower is better. The UK quotes mpg too, but with the larger imperial gallon, so a UK figure is about 20% higher than the US figure for the same car.

Formula

MPG = (current odometer − previous odometer) ÷ gallons added · L/100 km = 100 × liters ÷ km

Fill the tank completely both times so the fuel added equals the fuel burned since the last fill-up.

How to Improve Gas Mileage?

Drive smoothly. Hard acceleration and late braking waste the energy you just paid for. According to the US Department of Energy's fueleconomy.gov, speeding, hard acceleration and heavy braking can cut fuel economy by about 15–30% on the highway and 10–40% in stop-and-go traffic. Accelerate gently, look well ahead, and coast toward red lights instead of braking at the last moment.

Slow down on the highway. Aerodynamic drag grows with the square of speed, so fuel economy for most cars falls off quickly above about 50 mph (80 km/h). Every 5 mph above that point is roughly like paying an extra 20 to 30 cents per gallon. Cruise control helps hold a steady speed on flat roads.

Don't idle. An idling engine gets zero miles per gallon and can burn a quarter to half a gallon of fuel per hour, more with the air conditioning on. Modern engines need no warm-up beyond a few seconds; switch off if you will be stopped for more than about ten seconds outside traffic. Many new cars do this automatically with start-stop systems.

Lighten the load and reduce drag. An extra 100 pounds (45 kg) in the car cuts mileage by about 1%, and the effect is larger for smaller cars. Roof racks and rooftop cargo boxes add drag that can reduce highway fuel economy by several percent and up to about 25% for a large box at interstate speeds. Remove them when they are not in use, and use a rear-mounted carrier if you can.

Keep tires properly inflated. Under-inflated tires flex more and roll with more resistance; fueleconomy.gov estimates a loss of about 0.2% in mileage for every 1 psi drop in the average pressure of all four tires, and they wear out faster too. Check pressure monthly when the tires are cold, using the figure on the driver's door jamb rather than the maximum on the sidewall.

Maintain the car. Use the grade of motor oil the manufacturer recommends, replace worn spark plugs, and get the check-engine light looked at; a faulty oxygen sensor or misfiring cylinder can waste a lot of fuel. Keep the wheels aligned and the brakes free of drag.

Plan trips. Several short trips started from a cold engine use much more fuel than one longer trip covering the same distance, because engines are least efficient when cold. Combine errands, avoid rush hour where possible, and use the most efficient vehicle in the household for the longest drives. Using the air conditioner at low speeds and opening the windows at high speeds is a small but real saving.

Small changes add up. A driver covering 12,000 miles a year in a 25 mpg car buys 480 gallons of fuel; improving mileage by just 10% saves about 44 gallons, which is well over $100 a year at $3 per gallon, along with roughly 860 pounds of carbon dioxide.

Frequently asked questions