Two cars. One goes from 15 to 20 MPG. The other goes from 40 to 50 MPG. The second improvement is bigger in MPG terms — 10 rather than 5 — and it saves less than a third as much fuel.
That is not a trick. It is what happens when you measure a quantity by its reciprocal, and it is the single most important thing to understand about fuel economy units.
Economy and consumption are inverses
- Fuel economy — distance per unit of fuel. MPG, km/L. Higher is better.
- Fuel consumption — fuel per unit of distance. L/100km, gal/100mi. Lower is better.
Because they are reciprocals, you convert by dividing rather than multiplying:
| Conversion | Formula |
|---|---|
| US MPG → L/100km | 235.215 ÷ MPG |
| L/100km → US MPG | 235.215 ÷ (L/100km) |
| UK MPG → L/100km | 282.481 ÷ MPG |
| L/100km → UK MPG | 282.481 ÷ (L/100km) |
The formula is its own inverse, which is a neat consequence of the reciprocal relationship. The constants simply fold the gallon-to-litre and mile-to-kilometre conversions into one number — and there are two of them because there are two gallons.
The MPG illusion, with numbers
Take 100 miles of driving and work out the fuel used at each economy figure:
| US MPG | Gallons per 100 mi | L/100km | Saved vs. previous row |
|---|---|---|---|
| 10 | 10.00 | 23.5 | — |
| 15 | 6.67 | 15.7 | 3.33 gal |
| 20 | 5.00 | 11.8 | 1.67 gal |
| 30 | 3.33 | 7.8 | 1.67 gal |
| 40 | 2.50 | 5.9 | 0.83 gal |
| 50 | 2.00 | 4.7 | 0.50 gal |
Going 10 → 15 MPG saves more fuel than going 20 → 50 MPG. Replacing an old truck with a slightly less thirsty one usually saves more fuel than replacing an efficient car with a hybrid — which is genuinely counterintuitive if you are reading MPG figures, and obvious the moment you read consumption figures.
This is why US fuel economy labels now print gallons per 100 miles alongside MPG. The EPA added it specifically so that buyers could compare vehicles on a scale where equal differences mean equal savings.
Averaging: the error nobody notices
Drive 100 miles at 20 MPG, then 100 miles at 40 MPG. What is your average?
Not 30. You used 5 gallons for the first stretch and 2.5 for the second — 7.5 gallons over 200 miles, which is 26.7 MPG. Averaging MPG figures directly is simply wrong, because you would be averaging reciprocals.
Averaging L/100km works correctly over equal distances: 11.8 and 5.9 average to 8.8 L/100km, which converts back to 26.7 MPG. That property — that it adds and averages the way you expect — is a large part of why consumption units are preferred everywhere outside the US.
US MPG, UK MPG, and why UK cars look thriftier
An Imperial gallon is 4.546 L against the US gallon's 3.785 — 20% larger. A car travels the same distance either way, so its UK MPG figure is 20% higher than its US one for no physical reason at all.
| L/100km | US MPG | UK MPG |
|---|---|---|
| 4 | 58.8 | 70.6 |
| 5 | 47.0 | 56.5 |
| 6 | 39.2 | 47.1 |
| 8 | 29.4 | 35.3 |
| 10 | 23.5 | 28.2 |
| 12 | 19.6 | 23.5 |
So a car advertised at 47 MPG in Britain and 39 MPG in America may well be the same car with the same engine. Always establish which gallon before comparing.
Test cycles: the bigger variable
Unit conversion is exact. The figures being converted are not. Published economy comes from a standardised test, and the tests differ:
- EPA (US) — city, highway and combined, with adjustment factors intended to reflect real driving.
- WLTP (Europe, since 2017) — replaced the older NEDC, which was widely regarded as optimistic. WLTP figures are typically 10–20% worse than the NEDC ones for the same car.
- JC08 / WLTC (Japan) — different again.
A converted NEDC figure and an EPA figure for the same vehicle can differ by more than the entire US–UK gallon discrepancy. If you are comparing across markets, check the test standard as carefully as the unit.
Practical arithmetic
Once you are in L/100km, trip planning is a multiplication:
- Fuel for a trip = consumption × (distance ÷ 100). At 7.5 L/100km over 450 km: 7.5 × 4.5 = 33.75 L.
- Cost = that figure × price per litre.
From MPG the same calculation needs a division, and it is the division that hides the illusion. That asymmetry — one unit makes the important calculation easy, the other makes it awkward — is the practical argument for consumption over economy, independent of which system you grew up with.
Electric vehicles
EVs are rated in kWh per 100 km (or miles per kWh in the US), which is the same consumption-versus-economy split transplanted onto electricity. The US also publishes MPGe, converting the electricity into a notional gasoline equivalent at 33.7 kWh per gallon. MPGe inherits the MPG illusion exactly, so kWh/100km remains the clearer figure for comparing two EVs.