Gas Mileage Calculator
Enter how far you drove and how much fuel you used. Get your real-world economy in US mpg, UK mpg, L/100km and km/L — useful for cross-checking the dashboard reading or comparing cars across markets.
Fuel economy (US mpg)
30
- mpg (UK / Imperial)
- 36.03
- L per 100 km
- 7.84
- km per litre
- 12.75
- Fuel used (litres)
- 37.85
- Distance (km)
- 482.8
Distance is converted to km and fuel to litres using exact NIST factors (1 mi = 1.609344 km, 1 US gal = 3.785411784 L, 1 Imp gal = 4.54609 L). Economy is then expressed in all four common units: mpg-US, mpg-UK, L/100km and km/L.
How to use this calculator
Fill the tank, zero the trip meter, then drive normally and refill to the same level. Enter the trip distance (miles or kilometres) and the litres or gallons it took to refill — that is your real fuel use over that distance. The calculator converts everything to a canonical pair (km and litres) and reports your economy in all four common units. You can mix and match units freely: UK miles + litres at the pump is a perfectly normal combo.
How the calculation works
Distance is converted to kilometres and fuel to litres using exact NIST / UK Weights and Measures factors: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 Imperial gallon = 4.54609 L. From that single canonical pair the four economy figures fall out: mpg-US = miles ÷ US gallons, mpg-UK = miles ÷ Imperial gallons, L/100km = (litres ÷ km) × 100, km/L = km ÷ litres. The same drive shows up as different numbers in different units, but the underlying physics is one number.
Worked example
A 300-mile trip that consumed 10 US gallons: 300 ÷ 10 = 30 mpg-US. In UK terms that fuel is 10 × 3.785411784 = 37.854 L = 8.327 Imperial gallons, so 300 ÷ 8.327 = 36.0 mpg-UK. In metric, 300 mi = 482.80 km, so L/100km = (37.854 ÷ 482.80) × 100 = 7.84 L/100km, and km/L = 482.80 ÷ 37.854 = 12.76 km/L. All four numbers describe the same drive — they just suit different countries and dashboards.
Frequently asked questions
How do I measure my real fuel economy accurately?
Fill the tank to the first auto-stop click, note the trip-meter or odometer reading, then drive normally and refill to the same first click at the same pump style if possible. The litres / gallons it took on the second fill is the fuel used over that distance. Repeat over several tankfuls and average — a single tank is a snapshot, not a trend, and the auto-stop point can vary by a litre or two.
Why are mpg-US and mpg-UK different for the same car?
Because the gallon is different. A US gallon is 3.785411784 litres; an Imperial (UK) gallon is 4.54609 litres — about 20% larger. So a car that does 30 mpg-US does roughly 36 mpg-UK on the same fuel, even though nothing has changed mechanically. Always match the unit to the country where the figure was quoted: US window stickers use mpg-US, UK manufacturer brochures use mpg-UK.
Is L/100km better than mpg or worse?
Just different. L/100km is a consumption figure (lower is better — fewer litres per fixed distance). mpg is an economy figure (higher is better — more distance per fixed fuel). L/100km is also linear in fuel, which makes adding up trips easier; mpg is reciprocal, so a jump from 20 to 30 mpg saves much more fuel than a jump from 40 to 50 mpg. For comparing two cars on a long trip, L/100km is more intuitive.
Why does my real-world economy differ from the brochure figure?
Brochure figures are measured under a fixed lab cycle (WLTP in Europe, EPA in the US), which is more controlled than real driving. Cold starts, short trips, traffic, hills, heavy loads, low tyre pressure, roof boxes, and aggressive acceleration all reduce real-world economy. A 10–20% gap is normal; a 30%+ gap usually points at driving style or a maintenance issue.
Can I use this for an electric or hybrid vehicle?
For a regular hybrid, yes — measure litres of petrol used over a tank as for any car. For a plug-in hybrid, the petrol-only figure understates true running cost because the electric miles are not free either; a fair comparison needs both petrol economy and kWh per mile. For a pure EV this is not the right tool — you would want kWh/100km or miles/kWh, not mpg.
What counts as good fuel economy?
For a modern petrol family car, 35–45 mpg-US (42–54 mpg-UK, 5.2–6.7 L/100km) is solid; small economy cars and hybrids often top 50 mpg-US. Diesels typically beat petrol by 15–25% on the same body. SUVs and pickups commonly land at 18–25 mpg-US (22–30 mpg-UK, 9.4–13 L/100km). Anything significantly worse than the brochure suggests style or maintenance is the culprit.