Gas mileage calculator: how to find your real MPG

Most people check fuel economy once, when they buy the car, and then forget it. That is a pity, because MPG is one of the few numbers a car gives you every week for free. If you can calculate gas mileage from a fill-up, you can tell whether a new route costs more, whether winter is hurting you, and whether something on the car needs attention.

The gas mileage formula

The method most car guides recommend is the full-tank method. Kelley Blue Book describes it in six steps: fill the tank completely, record the mileage (a trip meter works, or note the odometer), drive normally, refill completely and record the exact gallons, work out the miles driven, then divide miles by gallons. Its example is 250 miles on exactly 10 gallons, which gives 25 mpg.

Why only the second fill counts: the gallons you add at the second stop are what the car used since the first one. The first fill just sets the starting line. If you want to skip the arithmetic, our fuel economy calculator does the division for you.

Turn MPG into cost per mile

Once you have MPG, the money side is one more division. Cost per mile = price per gallon ÷ MPG. Cost of a trip = miles × cost per mile.

A 300 mile trip in a 25 mpg car with gas at $3.50 uses 12 gallons and costs $42. The same trip at 30 mpg uses 10 gallons and costs $35. That gap is why a few miles per gallon matter over a year. Fuel is only one part of ownership, so see the true cost of owning a car for the rest.

Partial fills and missed fills

Real life does not always allow a perfect full tank every time. The fix is to keep a log and measure from one full tank to the next. Add up every gallon you bought in between, including partial fills, and divide the miles between the two full tanks by that total. If you forget to log a fill, that stretch is unreliable, so start counting again from the next full tank instead of guessing.

A simple log template

A notebook, a spreadsheet or an app all work. Each fill-up needs the same few columns:

Is the dashboard number good enough?

It is closer than many drivers assume, but it is still an estimate. In AAA's tests of 2018 to 2020 model year vehicles, in-dash displays were off by 2.3% (0.7 mpg) on average compared with lab-measured fuel economy. One vehicle overstated its economy by 6.4% (2.2 mpg) over the full test series, while the best one was essentially exact. AAA's results varied widely by vehicle, so you cannot assume yours is one of the accurate ones. Checking the display against a few tanks of your own arithmetic tells you how far to trust it.

Why your MPG changes

If your result moves from tank to tank, that is normal. Several things shift it, and the US government's fueleconomy.gov puts numbers on some of them:

• Speed. Gas mileage usually decreases rapidly at speeds above 50 mph, according to fueleconomy.gov. It estimates that slowing down by 5 to 10 mph improves fuel economy by an average of 7% to 14%.
• Aggressive driving. Speeding and rapid acceleration and braking can lower mileage by about 15% to 30% at highway speeds and about 10% to 40% in stop-and-go traffic, per the same source.
• Idling. An idling engine burns about a quarter to a half gallon of fuel per hour, depending on engine size and air conditioner use.
• Extra weight. An extra 100 pounds could reduce MPG by about 1%, and the effect is proportionally larger on smaller vehicles.
• Tire pressure. Proper pressure improves mileage by 0.6% on average and up to 3% in some cases. Each 1 psi drop in average tire pressure lowers mileage by about 0.2%.
• Wrong oil grade. Using a motor oil grade other than the one your manufacturer recommends can cost 1% to 2%.

These are fueleconomy.gov averages, not promises for your car. Your own log shows what actually happens to yours.

What a sudden drop can mean

Kelley Blue Book advises watching for unexpected drops in MPG, which it says can point to things like improper tire pressure, inefficient driving habits or a maintenance problem. Before you worry, rule out the easy causes: a different route, colder weather, a roof box, or a week of short trips. If the drop stays after those are ruled out, have the car looked at. A fuel log will not tell you what is wrong, but it tells you when it started, and that helps a mechanic.

Use the number

1. Calculate MPG for three or more tanks and average them. One tank can mislead.
2. Compare it with your car's rated figure, and with your own history, not with other people's cars.
3. Work out cost per mile, then use it to budget trips and monthly fuel.
4. Recheck after a change: new tires, a service, a different commute, or a season change.

If you want this done automatically, the fuel log in Badger stores each fill-up with gallons, price and odometer. For a deeper look at partial fills and the L/100 km conversion, read our guide to calculating your real fuel economy.

Common questions