How far does your fuel go?
Fuel economy tells you how efficiently a vehicle turns fuel into distance. In the US it is usually miles per gallon (mpg); in the UK it is mpg too, but with a larger gallon; in most other countries it is litres per 100 kilometres (L/100 km), where a smaller number is better.
This gas mileage calculator works out your real figure from a fill-up, converts it between all the common measures, and in the third mode turns a distance into fuel and money, so you can price a road trip or split the cost with friends.
How to use it
- Fill the tank to the top and reset the trip counter (or note the odometer).
- Drive as normal. At the next fill-up, fill to the top again and note how much fuel went in.
- Enter the distance and the fuel used, in miles and gallons or in kilometres and litres.
- For a trip, choose Trip fuel cost and enter the distance, your fuel economy and the price of fuel.
Measuring over several tanks gives a steadier figure than one, since a single fill-up can stop at a slightly different level.
The formulas
mpg = miles ÷ gallons
L/100 km = litres ÷ kilometres × 100
L/100 km = 235.215 ÷ mpg (US) = 282.481 ÷ mpg (UK)
trip cost = distance ÷ economy × price of fuelThe conversions use exact definitions: a mile is 1.609344 km, a US gallon is 3.785411784 litres and a UK gallon is 4.54609 litres. Because the UK gallon is about 20% larger, the same car shows a higher mpg figure in the UK.
Worked example: 350 miles on 12.5 gallons
- Fuel economy: 350 ÷ 12.5 = 28 mpg (US)
- In metric: 12.5 gallons is 47.32 litres, and 350 miles is 563.27 km
- 47.32 ÷ 563.27 × 100 = 8.4 L/100 km
The car does 28 mpg, which is 8.4 L/100 km, or 33.63 mpg in UK gallons. At that rate, a 420 mile trip uses 15 gallons, which costs $52.50 when fuel is $3.50 a gallon.
MPG to L/100 km
| mpg (US) | L/100 km | mpg (UK) |
|---|---|---|
| 15 | 15.68 | 18.01 |
| 20 | 11.76 | 24.02 |
| 25 | 9.41 | 30.02 |
| 30 | 7.84 | 36.03 |
| 35 | 6.72 | 42.03 |
| 40 | 5.88 | 48.04 |
| 50 | 4.7 | 60.05 |
The gains are not even. Going from 15 to 20 mpg saves almost 4 litres per 100 km, while going from 40 to 50 mpg saves just over 1. That is why L/100 km is a fairer way to compare fuel use between cars.
What lowers your mileage
- High speed: air drag rises sharply above highway speeds.
- Short trips and cold starts, before the engine warms up.
- Hard acceleration and braking.
- Under-inflated tyres, roof boxes and extra weight.
- Long idling. A speed, distance and time calculator helps plan a steadier journey.
Frequently asked questions
How do I calculate miles per gallon?
Divide the miles you drove by the gallons of fuel you used. If you drove 300 miles and filled up with 10 gallons, you got 30 mpg. For the most accurate figure, fill the tank to the top both times and use the trip counter between the two fill-ups.
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg figure, or 282.481 by the UK mpg figure. So 30 mpg (US) is 7.84 L/100 km. The two scales run in opposite directions: a higher mpg is better, while a lower L/100 km is better. The calculator shows both with every answer.
How do I work out the fuel cost of a trip?
Divide the trip distance by your fuel economy to get the fuel needed, then multiply by the fuel price. A 420 mile trip at 28 mpg needs 15 gallons, which is $52.50 at $3.50 a gallon. Choose Trip fuel cost above and enter any currency.
Why is UK mpg higher than US mpg?
The UK gallon is 4.54609 litres and the US gallon is 3.785411784 litres, so a UK gallon holds about 20% more fuel. The same car goes further on a UK gallon, which makes its mpg figure about 20% higher. The car is just as efficient either way.
What is a good fuel economy?
It depends on the vehicle, so the best yardstick is the official rating for your own model, printed on the window sticker or in the handbook. Heavy, tall vehicles use more fuel than small, low ones, and hybrids use the least. Real driving usually comes out a little below the rating.
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