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

Return fuel cost by route, petrol at 161.2p per litre

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.

Budget for the motorway premium

A trip estimate built on the national average of 161.2p assumes you fill up like the average driver. If your route plan says "services on the M6", it should not. Motorway service forecourts average 171.4p for petrol and 189.5p for diesel, a premium of roughly 10p a litre over every other forecourt.

What services fills add to a two-stop trip, September 2026

Tank size	Extra per services fill	Extra for two stops
50 litres	£5.25	£10.50
60 litres	£6.30	£12.60

On a £90 fuel budget that is an 11-14% planning error before you have even opened the calculator, and it compounds with the other classic mistakes - trusting the brochure mpg, forgetting the roof box, ignoring the extra weight of passengers and luggage. A realistic estimate uses your own observed consumption, ordinary-forecourt prices and a margin for the unusual; a lazy one can be out by a quarter.

For diesel cars the cash penalty is similar - services average 189.5p against 180.2p elsewhere - so the planning error is just as large whatever you put in the tank. The fix is identical either way: work the numbers at ordinary prices, then stick to them by filling before you join or at a junction town rather than between them.

Build the estimate on ordinary-forecourt prices, then honour it by filling before you join or at a junction town - the full comparison is in our motorway services guide.

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.

Should I budget extra for rural and island legs?

Yes. Rural routes with a single forecourt and a long delivery run are consistently dearer, and island pricing sits well above the national average - our data shows remote independents among the most expensive stations in the country. If a leg of the trip runs through sparse territory, budget it at a few pence above the national average rather than at it.

Figures updated 27 September 2026. Based on the UK average of 161.2p for petrol and 182.4p for diesel across 7,986 forecourts via the CMA open fuel price scheme. Route distances are approximate road miles. Cost per mile uses 4.546 litres per imperial gallon. Town-spread figures cover the 526 UK towns with four or more priced stations.