How to measure your real fuel economy
The dashboard read-out in most cars is an estimate and is often a little optimistic. The most reliable method is the full-tank method:
- Fill the tank completely and reset the trip meter (or note the odometer).
- Drive normally until you need fuel again.
- Fill the tank completely again and note the amount of fuel added and the distance driven.
- Enter both numbers in the calculator.
The fuel you added at the second fill-up is exactly what you used over that distance. Repeat over several tanks for a solid average, because a single tank can be skewed by a long motorway trip or a week of short city runs.
The formulas
US mpg = miles ÷ US gallons
UK mpg = miles ÷ imperial gallons
L/100 km = litres ÷ km × 100
km/L = km ÷ litres
The calculator converts everything internally (1 mile = 1.609 km, 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L), so you can mix units, for example miles and litres.
Example (US): you drove 320 miles and filled up with 11.2 US gallons. That is 28.6 mpg (US), 34.3 mpg (UK), 8.23 L/100 km or 12.15 km/L. At $3.40 per gallon your fuel costs $0.12 per mile.
Example (UK): 410 miles on 42 litres. 42 litres is 9.24 imperial gallons, so 44.4 mpg (UK), or 6.36 L/100 km.
Example (Europe): 640 km on 42.5 litres gives 6.64 L/100 km or 15.1 km/L.
What is good fuel economy?
| Vehicle type | Typical real-world US mpg | L/100 km |
|---|---|---|
| Pickup truck or large SUV | 15–22 | 15.7–10.7 |
| Midsize SUV | 22–30 | 10.7–7.8 |
| Midsize sedan | 28–36 | 8.4–6.5 |
| Compact car | 32–40 | 7.4–5.9 |
| Full hybrid | 45–55 | 5.2–4.3 |
Your figure depends heavily on driving style, speed, temperature, tyre pressure and load. A sudden drop in economy can point to under-inflated tyres, a dragging brake or a sensor problem.
Ways to use less fuel
- Keep tyres inflated to the pressure on the door sticker; under-inflation can cost several percent.
- Drive smoothly. Anticipate traffic, avoid hard acceleration and use cruise control on highways.
- Slow down a little. Going 65 instead of 75 mph saves noticeable fuel on long trips.
- Remove dead weight and roof racks or boxes when you don’t need them.
- Plan trips. A cold engine uses much more fuel, so combine short errands.
Compare your economy before and after a change with the percentage change calculator, and plan the cost of your next trip with the fuel cost calculator.
Tracking fuel economy over time
One tank tells you little; a few months of fill-ups tell you a lot. Write down the date, odometer reading and liters or gallons each time you fill up completely, or use a fuel tracking app. A sudden drop in fuel economy can point to underinflated tires, a dragging brake, a dirty air filter or a sensor problem, so it is a cheap early warning. Expect seasonal changes too: winter fuel blends, cold engines and heating can raise consumption by 10% to 20% compared with summer.
Frequently asked questions
How do I calculate my mpg?
Divide the miles driven since your last full tank by the gallons needed to fill up again. 300 miles on 10 gallons is 30 mpg.
How do I convert mpg to L/100 km?
Divide 235.21 by US mpg, or 282.48 by UK mpg. 40 US mpg is 5.88 L/100 km.
Why does my car use more fuel in winter?
Cold engines and cold air increase friction and drag, winter fuel blends contain slightly less energy, and heaters, seat heating and defrosting use extra power. A 10–20% drop in winter is normal.
Is the dashboard mpg accurate?
It is usually within a few percent but often slightly optimistic. The full-tank method is more accurate over time.
What is the difference between US and UK mpg?
A UK (imperial) gallon is about 20% larger than a US gallon, so the same car shows a higher mpg figure in the UK.
Last reviewed: 2026-10-06. Results are estimates for information only.