How to Use This Tool
Put in the distance, the car's economy and the fuel price. Tick round trip if you are coming back, and set the number of people if you are splitting it.
MPG makes savings look wrong
Miles per gallon measures distance per unit of fuel, and the thing you pay for is fuel per unit of distance. Because it is upside down, equal-looking improvements are wildly unequal.
Over 10,000 miles:
- 10 to 20 MPG takes you from 1,000 gallons to 500 — a saving of 500.
- 30 to 50 MPG takes you from 333 gallons to 200 — a saving of 133.
Twice the MPG improvement, a quarter of the fuel saved. This is known as the MPG illusion, and it has a practical consequence: replacing the least efficient vehicle in a household almost always saves more than upgrading the efficient one, even though the MPG jump looks smaller.
It is also why most of the world quotes litres per 100 km, which is the right way up. One litre per 100 km saved is the same amount of fuel whatever you started from.
Which gallon
A US gallon is 3.785 litres and an imperial gallon is 4.546. The same car is quoted at 30 MPG in the United States and 36 MPG in the United Kingdom without anything about it changing.
So a figure copied from an American review into a British calculator overstates economy by about 20%, and the trip comes out too cheap. The unit selector here is not a formality — pick the one your figure came from.
What actually changes the number
Roughly in order of size, for a trip you have already decided to make:
- How many people are in the car. Four passengers quarters the cost per head. Nothing else comes close.
- Speed. Air resistance rises with the square of speed, so the last 10 mph costs far more than the first. Most cars are noticeably thirstier above roughly 60–70 mph.
- Where you fill up. Motorway services are routinely the most expensive fuel on the route; a short detour before joining is often worth more than any driving technique.
- Load and roof boxes. A roof box is a large aerodynamic penalty at motorway speed and costs nothing when it is off the car.
- Tyre pressure. Small, free, and worth checking anyway.
Fuel is not the cost of driving
This calculates fuel only. The real cost per mile also includes tyres, servicing, depreciation and the share of insurance that mileage drives. Published figures vary widely, but fuel is typically well under half of it.
That matters when comparing driving with a train or a flight: fuel alone makes driving look cheaper than it is, and adding a passenger changes the comparison more than any of the arithmetic.
