Fuel for one trip, and what it costs
Work out the fuel first, then multiply by the pump price. A 500-mile trip at 30 MPG needs 16.7 gallons; at $3.50 a gallon that is $58.35. In metric, 800 km at 8 L/100km needs 64 litres; at $1.60 a litre that is $102.40.
MPG and L/100km measure the same thing backwards
This is the one trap. A bigger MPG number is good; a bigger L/100km number is bad. They are not a straight scale of each other, so you convert with a fixed constant: divide 235.215 by whichever figure you hold.
Doubling MPG halves the litres burned per 100 km. The efficient car uses half the fuel over the same distance.
Common fuel-economy figures
| MPG (US) | L/100km | km/L | Typical vehicle |
|---|---|---|---|
| 20 | 11.8 | 8.5 | Pickup truck / large SUV |
| 25 | 9.4 | 10.6 | Mid-size SUV |
| 30 | 7.8 | 12.8 | Family sedan |
| 35 | 6.7 | 14.9 | Compact car |
| 50 | 4.7 | 21.3 | Hybrid |
Convert either way with 235.215 ÷ MPG = L/100km, and 235.215 ÷ L/100km = MPG. For km/L, divide MPG by 2.352.
Two ways to read the number, and why the pump differs
- Rated vs real. Divide distance by the fuel you actually put in to get your true economy, then compare it against the sticker figure — real-world numbers usually run lower.
- What eats fuel. Aggressive driving adds 15–30%, running the AC 5–25%, cold weather a few percent, and every 100 lb of load about 2%.
- US vs imperial gallon. A US gallon is 3.785 litres; a UK gallon is 4.546 litres. A "40 MPG" UK figure is not the same trip cost as 40 MPG in the US.
Common questions
How do I work out the fuel for a trip?
In MPG, divide the miles by the MPG to get gallons: 500 miles at 30 MPG is 16.7 gallons. In metric, multiply the distance by the consumption and divide by 100: 800 km at 8 L/100km is 64 litres. Multiply the fuel by the pump price for the cost.
Why can I not just compare MPG to L/100km directly?
They run in opposite directions. Higher MPG is better, but lower L/100km is better. 30 MPG is the same as 7.84 L/100km, and a car at 6 L/100km beats one at 8. To swap between them, divide 235.215 by the number you have.
How much does one gallon of petrol add in CO2?
About 8.9 kg, which is heavier than the fuel itself because each carbon atom picks up two oxygen atoms from the air when it burns. One litre works out to roughly 2.31 kg.
