Measure It at the Pump
The most accurate way to measure your real fuel economy is the fill-to-fill method:
- Fill the tank and note the odometer.
- Drive normally until the next fill-up, then fill it again.
- Enter the distance since the last fill and the gallons or litres you just added.
- Optionally add the fuel price to see trip cost and cost per 100 km or 100 miles.
You get MPG (US), MPG (UK), L/100km and km/L at once, with the working shown. Averaging over a whole tank smooths out cold starts, traffic and motorway stretches. If you only know the route, the Distance Calculator gives the distance to plug in.
Four Units, One Ratio
Every unit comes from the same two numbers, distance and fuel. What differs is which one goes on top, and whether a bigger number is better:
- MPG (US) = miles ÷ US gallons. Higher is better. A US gallon is 3.78541 litres.
- MPG (UK) = miles ÷ imperial gallons. Higher is better. An imperial gallon is 4.54609 litres, about 20% bigger, so UK figures read 1.20095 times higher for the same car.
- L/100km = litres ÷ km × 100. Lower is better, because it measures fuel per distance.
- km/L = km ÷ litres. Higher is better, and km/L = 100 ÷ L/100km.
The one constant to remember: MPG (US) × L/100km = 235.215, so each converts to the other with one division.
Conversion Table
| MPG (US) | MPG (UK) | L/100km | km/L |
|---|---|---|---|
| 15 | 18.0 | 15.68 | 6.38 |
| 20 | 24.0 | 11.76 | 8.50 |
| 25 | 30.0 | 9.41 | 10.63 |
| 30 | 36.1 | 7.84 | 12.75 |
| 35 | 42.1 | 6.72 | 14.88 |
| 40 | 48.1 | 5.88 | 17.00 |
| 50 | 60.1 | 4.70 | 21.25 |
| 60 | 72.1 | 3.92 | 25.51 |
Which Unit Your Country Uses
- United States: MPG with US gallons, rated by the EPA on a combined 55% city, 45% highway cycle. Real-world results are often 10–20% lower.
- United Kingdom and Ireland: MPG with imperial gallons, tested under WLTP since 2018. Divide by 1.20095 to compare with a US figure.
- Canada, Europe and Australia: L/100km, from Natural Resources Canada's annual guide and WLTP figures in Europe.
- India, Japan and the Philippines: km/L, often written kmpl in India, where ARAI certifies the figures.
What Counts as Good
It depends on the vehicle. Good figures by class: full-size trucks and SUVs 20–22 MPG (10.7–11.8 L/100km), midsize crossovers 26–30 (7.8–9.1), midsize sedans 30–38 (6.2–7.8), compacts 35–45 (5.2–6.7), hybrids 50–58 (4.1–4.7) and motorcycles 45–70. Broadly, above 35 MPG (below 6.7 L/100km) is doing well, and below 25 MPG (above 9.4) starts to cost real money.
When comparing improvements, watch the direction. Going from 26 to 32 MPG is a 23% rise in MPG, but fuel used per 100 km falls from 9.05 to 7.35 litres, an 18.75% drop, which the Percentage Decrease Calculator works out. To plan fuel for a long trip, get the distance first. To weigh driving against flying, the Flight Distance Calculator and the flight distance chart for major US cities give the air distance.
Five Fill-Ups, Converted
| Situation | Measured | Converted |
|---|---|---|
| Camry Hybrid, US: 332 miles on 9.4 US gal | 35.32 MPG | 6.66 L/100km, 15.0 km/L (EPA combined rating is 37 MPG) |
| Autobahn diesel: 480 km on 38.4 L | 8.0 L/100km | 12.5 km/L, 29.4 MPG US, 35.3 MPG UK (high speed costs fuel) |
| UK motorcycle: 180 miles on 3.2 imperial gal | 56.25 MPG UK | 46.84 MPG US, 5.02 L/100km |
| Hatchback in Bengaluru: 620 km on 48 L | 12.92 km/L | 7.74 L/100km, 30.4 MPG US (city driving, well below the certified figure) |
| Car A at 31 MPG (US) vs Car B at 7.2 L/100km | A = 7.59 L/100km | B = 32.67 MPG, so B is slightly more efficient |
How the Calculator Is Verified
A US gallon holds 3.78541 liters; an imperial gallon holds 4.54609 liters, about 20 percent more. The calculator converts your distance and fuel inputs to a single km-per-liter figure first, then multiplies that by each gallon size separately to produce MPG (US) and MPG (UK) side by side, so a car rated at 40 MPG in the US figure and 48 MPG in the UK figure on this page is the same actual fuel economy, just expressed against two differently sized gallons. Both numbers stay visible regardless of which distance or fuel unit you picked in the input fields.
Frequently Asked Questions
Does driving speed affect fuel efficiency?
Yes, significantly. Most petrol and diesel cars peak efficiency at 80-95 km/h (50-60 mph). Above that, aerodynamic drag increases with the square of speed, so fuel consumption rises sharply. Driving at 120 km/h typically uses 20-30% more fuel than driving at 90 km/h. Cold weather also reduces efficiency because engines take longer to warm up and battery-powered systems draw more power.
How do I improve my car fuel efficiency?
The biggest gains come from driving style: smooth acceleration, anticipatory braking, and avoiding idling. After that: keep tyres at the correct pressure (under-inflation adds rolling resistance), remove roof racks when not in use, and service the engine on schedule. A dirty air filter, worn spark plugs, or low tyre pressure can each cost 2-5% in efficiency. Trip planning helps too: combining short errands into one warm-engine run is far more efficient than multiple cold starts.
References
- US Department of Energy, Fuel Economy Guide, fueleconomy.gov: source for EPA MPG ratings and fuel economy testing methodology for US-market vehicles.
- Natural Resources Canada, Fuel Consumption Guide, nrcan.gc.ca: annual guide covering L/100km ratings for all vehicles sold in Canada.
- European Environment Agency, CO2 Emissions from New Passenger Cars, eea.europa.eu: data on EU fleet average fuel efficiency and CO2 emissions trends under the WLTP test cycle.