⛽ Fuel economy
MPG to L/100 km: why the conversion isn't linear
Both numbers describe how much fuel a car uses. One is distance per volume, the other is volume per distance — so converting between them is a division, not a multiplication, and equal-sounding improvements are not equal savings.
Two directions, one quantity
Miles per gallon answers "how far on a gallon?" Litres per 100 km answers "how much for a fixed distance?" Each is the reciprocal of the other, scaled by the units. That single fact explains almost every confusing thing about fuel economy figures.
Because the relationship is reciprocal, it plots as a hyperbola. Doubling mpg halves consumption, but adding a fixed number of mpg does something different at every point on the curve. A 5 mpg improvement on a 20 mpg car is a large saving; the same 5 mpg on a 45 mpg car is almost nothing.
The exact factors
Three definitions do all the work. A US gallon is exactly 3.785411784 L (231 cubic inches). An imperial gallon is exactly 4.54609 L. A mile is exactly 1.609344 km. From those:
L/100 km = 235.2146 ÷ US mpg and L/100 km = 282.4809 ÷ imperial mpg.
The US constant comes from 100 ÷ 1.609344 = 62.1371 miles in 100 km, divided by mpg to get gallons, then multiplied by 3.785411784. The imperial constant uses the larger gallon, which is why a car looks about 20% more efficient in British mpg than in American mpg for the same real consumption.
| US mpg | Imperial (UK) mpg | L/100 km | km per L |
|---|---|---|---|
| 15 | 18.0 | 15.68 | 6.38 |
| 20 | 24.0 | 11.76 | 8.50 |
| 30 | 36.0 | 7.84 | 12.75 |
| 40 | 48.0 | 5.88 | 17.01 |
| 50 | 60.1 | 4.70 | 21.26 |
| 60 | 72.1 | 3.92 | 25.51 |
| 80 | 96.1 | 2.94 | 34.02 |
Imperial mpg here is the modern imperial gallon, the one used in UK advertising and in older British road tests. The historic "Imperial gallon" and the US "dry gallon" are not relevant to fuel and should not appear in any calculation.
Why "5 mpg better" is not one number
Take two cars, both driven 20,000 km a year. Improving the first from 20 to 25 US mpg takes it from 11.76 to 9.41 L/100 km — a saving of 2.35 L per 100 km, or 470 L a year. Improving the second from 40 to 45 mpg takes it from 5.88 to 5.23 L/100 km — 0.65 L per 100 km, or 130 L a year.
Same headline improvement, 3.6 times the fuel. This is why fleet rules and buyer advice increasingly quote L/100 km or gallons per 100 miles: those units are proportional to fuel burned, so improvements add up the way people expect.
| From → to | L/100 km before | L/100 km after | Litres saved per year |
|---|---|---|---|
| 20 → 25 mpg | 11.76 | 9.41 | 470 |
| 25 → 30 mpg | 9.41 | 7.84 | 314 |
| 30 → 35 mpg | 7.84 | 6.72 | 224 |
| 35 → 40 mpg | 6.72 | 5.88 | 168 |
| 40 → 45 mpg | 5.88 | 5.23 | 130 |
| 50 → 55 mpg | 4.70 | 4.28 | 84 |
MPGe, and why it is not mpg
Electric cars cannot be measured in gallons, so the US EPA uses a gasoline-equivalent: 33.7 kWh of electricity is treated as equal to one US gallon of petrol, based on the average energy content of petrol in the US test programme. MPGe is therefore 33.7 ÷ consumption in kWh per mile.
A car that uses 0.30 kWh per mile (about 186 Wh/km) scores 33.7 ÷ 0.30 = 112 MPGe. At 0.25 kWh per mile it scores 135 MPGe. The figure is useful for comparing cars, but it hides two things: charging losses of roughly 10% between the wall and the battery, and the fact that electricity and petrol are priced per different unit.
To compare running costs, ignore MPGe and multiply: litres per 100 km × price per litre, and kWh per 100 km × price per kWh. That is the only arithmetic that answers "which is cheaper to run where I live".
Cost per distance, done properly
In imperial markets the sum is simple: cost per mile = price per gallon ÷ mpg. At $3.50 a gallon and 32 mpg, that is 10.9 cents a mile. In metric markets: cost per 100 km = L/100 km × price per litre. At 7.5 L/100 km and €1.75 a litre, that is €13.13 per 100 km, or 13.1 cents a km.
An easy sanity check when comparing two markets: 1 L/100 km is roughly 235 US mpg, so a car at 8 L/100 km is about 29 mpg US, and a car at 5 L/100 km is about 47 mpg US.
Official figures versus the road
Test cycles are repeatable by design, which makes them unrepresentative by design. The US EPA cycle and the European WLTP both run without traffic, wind, hills, passengers or a cold winter morning. Expect real consumption to be 10–25% above the label, with the worst cases being short urban trips in cold weather, where the engine never reaches operating temperature.
Things that move the number more than most drivers expect: tyre pressure (10% under-inflation costs roughly 1–2%), roof bars and bike racks (5–15% at motorway speed, because aerodynamic drag rises with the square of speed), air conditioning (5–20% in town), and aggressive acceleration followed by hard braking.
Rounding, and how the converters here handle it
A conversion shown to one decimal place is fine for a dashboard display but not for comparing two cars a tenth apart. The site's fuel economy converter carries full precision internally and rounds only at display, so repeated conversions do not accumulate rounding drift.
One practical caveat: if you convert mpg to L/100 km, round, then convert back, you will not always land on the original number. That is rounding, not an error in the factor — keep more digits in the middle of a calculation than you intend to show at the end.
Frequently asked questions
Sources
- NIST Special Publication 811 — definitions of the US gallon and the mile.
- UK Weights and Measures Act 1985 and the Weights and Measures (Units of Measurement) Regulations — imperial gallon of 4.54609 L.
- US Department of Energy, fueleconomy.gov — MPGe and the 33.7 kWh gasoline-equivalent.
- US EPA and UN ECE WLTP test-cycle documentation — what the official figures do and do not include.
- US DOE Fuel Economy Guide — effects of tyre pressure, accessories, speed and temperature.