Auto & travel
Fuel cost calculator
The number worth knowing is not the tank, it is the trip. Distance, economy and price give you the cost of the journey, the cost per mile, and what each passenger owes.
Two ways of saying the same thing, and why one is better
MPG measures distance per unit of fuel. Litres per 100 km measures fuel per unit of distance. They describe the same physical fact, but MPG is the inverted one, and inversion makes it misleading.
Consider two upgrades. Going from 10 to 15 MPG saves 3.3 gallons per 100 miles. Going from 30 to 50 MPG — a far more impressive-sounding jump — saves only 1.3 gallons over the same distance.
| Change | MPG gained | Gallons saved per 100 mi |
|---|---|---|
| 10 → 15 MPG | 5 | 3.33 |
| 20 → 25 MPG | 5 | 1.00 |
| 30 → 50 MPG | 20 | 1.33 |
This is the “MPG illusion” documented by Larrick and Soll in Science in 2008. Replacing the least efficient vehicle in a fleet saves far more fuel than making an already efficient one better, but MPG figures make the opposite look true. Litres per 100 km, used across most of Europe, does not have this problem — the number is directly proportional to fuel burned.
Gallons are not gallons
A US gallon is 3.785 litres; an imperial gallon is 4.546. That makes a UK car quoted at 40 MPG equivalent to about 33 US MPG for the same real efficiency. Comparing figures across the Atlantic without converting overstates British fuel economy by 20%.
Why your car never matches the sticker
Official economy figures come from standardised laboratory cycles. Real driving differs in every way that matters. Independent testing consistently finds a gap of 10–20% against the newer WLTP standard, and considerably more against the older NEDC cycle it replaced.
- Speed — aerodynamic drag rises with the square of speed, so power needed rises with the cube. Driving at 80 mph instead of 65 can cost a fifth of your economy.
- Cold — short winter trips can be 20–30% worse, since an engine below operating temperature runs rich and the cabin heater draws energy.
- Load — a roof box costs 10–25% at motorway speed even when empty. Removing it when not in use is one of the cheapest efficiency gains available.
- Tyres — under-inflation raises rolling resistance measurably and wears tyres faster.
The reliable way to know your real figure is to measure it: fill the tank, note the odometer, drive normally for a few hundred miles, fill up again and divide.
Fuel is not the cost of driving
Fuel is the visible cost, which is why people use it to compare journeys, but it is a minority of the true cost per mile. Depreciation, insurance, maintenance, tyres and tax typically add up to more. The US federal mileage reimbursement rate is set well above fuel cost alone for exactly this reason. For deciding whether to drive or take the train, comparing train fare against fuel alone understates the car considerably.
Common questions
How do I calculate fuel cost for a trip?
Divide the distance by your fuel economy to get the volume of fuel, then multiply by the price. For 300 miles at 32 MPG with fuel at $3.50: 300 ÷ 32 = 9.4 gallons, × $3.50 = $32.81. In metric, multiply distance by L/100 km and divide by 100 to get litres.
What is 30 MPG in litres per 100 km?
About 7.8 L/100 km if that is a US gallon figure, or 9.4 L/100 km if imperial. The conversion is 235.215 divided by US MPG, or 282.481 divided by imperial MPG. Note that a lower L/100 km number means better economy, the opposite direction from MPG.
Why is my real fuel economy worse than advertised?
Official figures come from a standardised test cycle that is smoother, gentler and warmer than real driving. Independent testing finds real-world results typically 10 to 20 percent below the WLTP figure. Motorway speeds, cold weather, short trips, roof racks and city traffic all widen the gap.
Is it cheaper to drive or fly?
For one person over a long distance, flying is often cheaper once you count the full cost of driving rather than fuel alone. For three or four people, driving usually wins, since fuel cost is fixed regardless of occupancy while airfares multiply per passenger. Compare fuel plus tolls plus a realistic per-mile allowance for wear against the total fare including baggage and transfers.