Gas Mileage Calculator

Fill completely, drive, fill completely again

The arithmetic on this page is a division. Everything that makes the answer trustworthy happens at the pump, before you type anything, and it comes down to one idea: if the tank is at exactly the same level at the start and at the end, then the fuel you just put in is exactly the fuel you burned.

1. Fill the tank completely. Not to the first click of the nozzle and not to a round number on the display — to the same definite stopping point you will use again.
2. Reset the trip meter, or write down the odometer reading. Either works; this page takes both.
3. Drive normally. Do not save it for a good week — a measurement of your best driving is a measurement of nothing you actually do.
4. Fill completely again, to the same point, and keep the receipt. The litres or gallons on it, and the distance since the last fill, are the two numbers this page needs.

A partial fill at either end makes the result meaningless. It is the most common mistake by a wide margin, and it is dangerous precisely because the answer still looks reasonable. If the first fill went to the brim and the second stopped early, you have divided a real distance by less fuel than you burned, and the mpg comes out flattering. There is no correction for it. Start again.

One more thing the arithmetic cannot see: a single tank is a noisy sample. City versus highway mix is worth many miles per gallon on its own; cold weather costs several percent through longer warm-ups, denser air and winter fuel; tyres a few psi down cost a percent or two. Average three or four consecutive tanks before you trust a figure, and when you compare two periods, compare like seasons.

How the four units relate

One measurement, four ways of saying it. The first three are the same fraction with different units on the bottom; the fourth is that fraction turned upside down.

Because L/100 km is a reciprocal, equal steps in it are not equal steps in economy. Going from 10 to 9 L/100 km saves 100 litres over 10,000 km; going from 5 to 4 saves 200. This is the reason consumption-per-distance is the better unit for deciding what to buy, and distance-per-fuel is the more intuitive one for describing a car.

The annual projection uses the rate, not the tank: yearly distance divided by the measured economy, then multiplied by the price. Scaling up the cost of the tank you just bought would fix the price you happened to pay that day into the whole year.

320 miles on 11.5 US gallons

320 ÷ 11.5 = 27.83 US mpg. In imperial gallons the same 11.5 US gallons is 9.58, so the car is 33.42 mpg in UK terms — the same car, a 20% higher number, because the gallon is 20% bigger. Metrically it is 515.0 km on 43.53 litres: 11.83 km/L, or 8.45 L/100 km.

With fuel at $3.50 a US gallon, that tank cost $40.25 and each mile cost 12.58 cents. Carry the rate forward over 12,000 miles a year and it is 431.25 gallons and $1,509.38 of fuel — which is the figure worth knowing, because it is the one that changes when you change cars, change tyres, or stop making the short cold trips that cost the most per mile.

If the dashboard on that car was showing 29 mpg while the pump said 27.83, nothing is broken. That is a 4% optimism, squarely inside the usual range for a trip computer, and it will stay about that size on the next tank too.

Three fuel tools, and the one question each answers

• This page — use it to find out what your car actually does. It turns one honest fill-up into an economy figure, in every unit at once.
• Fuel cost calculator — use it next, with the number this page gives you. It prices a specific trip, splits it between passengers, and shows what the same drive would cost in a different car.
• Fuel economy converter — use it when the only problem is units. It converts a figure you already have between US mpg, imperial mpg, km/L and L/100 km, and does nothing else.

What this calculator assumes

• Both fills stopped at the same level. Nothing in the arithmetic can detect that they did not, and no result on this page is meaningful if they did not.
• The distance is the distance covered on the fuel you are entering. If the trip meter was reset a fill late, or the odometer reading came from the wrong day, the answer is wrong by exactly that much.
• The odometer is accurate. Most read 1–3% optimistic from the factory, and non-standard tyre sizes shift it further, which flatters the economy figure by the same proportion.
• One tank represents your driving. It usually does not — see the method note above.
• The annual projection holds this economy and this fuel price for a whole year. Both move; the projection is a rate carried forward, not a forecast.
• The currency symbol is cosmetic. The arithmetic never touches it, so a price in pence per litre gives an answer in pence.

Gas mileage FAQ

Sources and review notes

1. US DOE and EPA, fueleconomy.gov — the factors that move real-world economy away from the test-cycle rating
2. NIST Special Publication 811 — the exact definitions used here for the mile, the US gallon and the imperial gallon
3. US EPA — how the published fuel economy figures are tested and what the test cycles cover

The conversion factors behind this page are exact by definition rather than measured, so the arithmetic does not go stale. What can go stale is the measurement you feed it: an economy figure is a snapshot of a car, a season and a driving mix, and it is worth redoing a few times a year rather than trusting one good tank.