How to Calculate Fuel Cost for a Road Trip
Multiply your distance by your cost per mile - and your cost per mile is (4.546 ÷ mpg) × price per litre. At the current UK average of 161.2p, a 40mpg car costs 18.3p a mile. A 300-mile trip is therefore about £55 each way. The two things people get wrong are using the brochure mpg and forgetting the return leg.
The calculation
There are 4.546 litres in an imperial gallon, so:
Cost per mile = (4.546 ÷ mpg) × price per litre in pounds
Trip cost = cost per mile × total miles
At 45mpg and 161.2p: (4.546 ÷ 45) × 1.612 = 16.3p per mile. A 250-mile journey costs £40.70, or £81.40 return.
Common trips, costed
| Route | Return miles | 35 mpg | 45 mpg | 55 mpg |
|---|---|---|---|---|
| London to Manchester | 400 | £83 | £65 | £53 |
| London to Edinburgh | 800 | £167 | £130 | £106 |
| Birmingham to Cornwall | 500 | £104 | £81 | £66 |
| Leeds to London | 400 | £83 | £65 | £53 |
| Cardiff to Glasgow | 800 | £167 | £130 | £106 |
Diesel costs more per litre at 182.4p but usually returns better economy on a long run, so the totals often land close. Our fuel cost calculator will work it out with live prices for your exact route and car.
Four things that make the estimate wrong
Using the manufacturer's mpg
Official WLTP figures are optimistic in real use. Long motorway runs at a steady 65mph often get close to them, but a loaded car with roof bars will not. Use your own measured figure if you have it.
Forgetting the load
A full car with luggage, or a roof box, meaningfully increases consumption. A roof box can cost 10% to 20% at motorway speeds because of drag, which is worse than the weight.
Ignoring where you fill up
This is the big one on a long trip. Filling at motorway services averages 171.4p against 160.9p elsewhere - a 10.5p premium. On a journey needing two 55-litre fills, that is £11.55 of avoidable cost. See are motorway service station prices higher.
Speed
Fuel consumption rises sharply above about 60mph because aerodynamic drag increases with the square of speed. The Energy Saving Trust puts the difference between 70mph and 80mph at around 10% to 15% more fuel.
Planning where to fill
- Start with a full tank from a cheap local station rather than filling en route.
- Plan one refuelling town roughly two thirds of the way, and check prices there before you leave using our regional pages.
- Treat services as a top-up, not a fill, if you have to stop.
- Fill before the return leg in the cheaper area, not at the last services on the way home.
Frequently asked questions
Should I include the return journey?
Yes, unless someone else is driving back. It sounds obvious but it is the single most common mistake in trip budgeting - people cost the outbound leg and are surprised at the total.
How do I find my real mpg?
Fill the tank completely, reset the trip meter, drive normally, then fill completely again. Divide the miles by the litres added and multiply by 4.546. Do it over a few hundred miles for a reliable figure.
Is it cheaper to drive slower?
Up to a point. Efficiency generally peaks somewhere between 45 and 60mph. Driving at 55 on a motorway will use less fuel than 70, though it will cost you time and is not always practical or safe in traffic flow.
Does air conditioning matter on a long trip?
Less than people think at motorway speeds. At high speed, opening windows creates enough drag that air conditioning is often the more efficient choice. In town the reverse is true.
Find the cheapest fuel near you
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