Miles per gallon and litres per 100 kilometres measure the same thing from opposite directions: one is distance per unit of fuel, the other is fuel per unit of distance. Because they are reciprocals, the conversion is not a straight proportion — and that quirk has a practical consequence most drivers never notice. Improving a thirsty car saves far more fuel than the same MPG gain on an efficient one.
The conversion
L/100km = 235.2146 ÷ MPG (for US gallons)
MPG = 235.2146 ÷ L/100km
The constant 235.2146 comes from 100 km ÷ (1.609344 km/mi × 3.78541 L/US gal). If you are working with UK (imperial) gallons, which are about 20% larger, the constant is 282.48 instead. US MPG figures are therefore lower than UK MPG for the same car — a genuinely confusing point when reading reviews from both markets.
Worked example: 30 MPG (US) → 235.2146 ÷ 30 = 7.84 L/100km. And 6.0 L/100km → 235.2146 ÷ 6.0 = 39.2 MPG.
Reference table
| MPG (US) | L/100km | MPG (UK) | Typical vehicle |
|---|---|---|---|
| 15 | 15.68 | 18.0 | Full-size pickup |
| 20 | 11.76 | 24.0 | Large SUV |
| 25 | 9.41 | 30.0 | Crossover |
| 30 | 7.84 | 36.0 | Mid-size sedan |
| 35 | 6.72 | 42.0 | Compact car |
| 40 | 5.88 | 48.0 | Small hatchback |
| 45 | 5.23 | 54.0 | Diesel compact |
| 50 | 4.70 | 60.0 | Standard hybrid |
| 60 | 3.92 | 72.0 | Efficient hybrid |
The Fuel Economy Converter converts in either direction and handles US versus UK gallons explicitly, so you do not have to remember which constant applies.
Why the scale is not linear — and why it matters
Here is the part that surprises people. Take two upgrades that each add 10 MPG:
- 20 → 30 MPG: 11.76 → 7.84 L/100km, saving 3.92 L per 100 km.
- 40 → 50 MPG: 5.88 → 4.70 L/100km, saving 1.18 L per 100 km.
Same ten-point MPG gain, but the first saves more than three times the fuel. Over 15,000 miles a year, the 20→30 improvement cuts roughly 165 gallons; the 40→50 improvement cuts about 50. This is why fuel-economy policy and fleet decisions target the worst performers first, and why L/100km is arguably the more honest scale: it is proportional to fuel consumed, so equal steps on the scale mean equal fuel saved.
The same logic applies to your own driving. Fixing an under-inflated tyre or clearing 200 lb of junk from the trunk helps a gas-guzzler far more than it helps a hybrid, in absolute litres.
Reading real-world figures
Official ratings are measured under controlled conditions and usually beat what drivers see. Cold weather, short trips, high speeds and aggressive acceleration all widen the gap, and a roof rack can cost several MPG on its own. Treat the sticker as a comparison tool between vehicles rather than a prediction of your own fuel bill. If you want the real number, fill the tank, reset the trip meter, and divide miles driven by gallons added at the next fill.
How to measure your own fuel economy accurately
The dashboard display is convenient but often optimistic by a few percent. For a figure you can trust, use the full-tank method:
- Fill the tank until the pump clicks off, and reset the trip meter.
- Drive normally until the tank is at least half empty — a longer distance reduces the error from the exact fill level.
- Refill at the same station and ideally the same pump, again to the first click.
- Divide miles driven by gallons shown on the receipt.
Repeat over three or four tanks and average the result. Filling to the first click consistently matters more than people expect: topping off adds variable amounts and makes the second measurement wrong. Using the same pump removes calibration differences between pumps.
Then convert: a car returning 32.5 MPG is 235.2146 ÷ 32.5 = 7.24 L/100km. Tracking this over time is genuinely useful, because a drop of more than about 10% with no change in driving usually points to something concrete — under-inflated tyres, a dragging brake, a failing oxygen sensor, or simply a roof rack that was left on.
Key Takeaways
L/100km = 235.2146 ÷ MPG (US), and MPG = 235.2146 ÷ L/100km; use 282.48 for UK gallons. Because the scales are reciprocal, a 10 MPG gain is worth far more on an inefficient car than an efficient one — 20 to 30 MPG saves over three times the fuel of 40 to 50 MPG. Compare vehicles on L/100km when you care about actual fuel burned.