Fuel Cost Calculator

The unit that actually helps is pence per mile

Fuel is priced per litre and economy is quoted in miles per gallon, which is why working out what a journey costs is more awkward than it should be. The conversion needs the imperial gallon, 4.54609 litres, not the US gallon of 3.785 that most online converters default to. Getting that wrong overstates your fuel economy by about 20%.

Pence per mile collapses all of it into one number you can actually use. In the example below, a car doing 45 mpg at 139.9p a litre costs 14.1p a mile. That single figure prices any journey, settles whether driving or the train is cheaper, and tells you what a commute really costs.

It also makes the mileage rates make sense. HMRC's approved rate for business mileage in your own car is 55p a mile for the first 10,000 miles in 2026/27 (45p before April 2026) and 25p after that, and those are deliberately set above fuel cost alone because they are meant to cover wear, servicing, insurance and depreciation too.

Economy moves the number far more than fuel price does

People watch the pump price and barely notice the car. On the same 13,000 miles a year, a car returning 45 mpg costs £1,837.33 in fuel; one returning 30 mpg costs £2,755.99. That is £918.66 a year, from the car alone, at an identical pump price.

For comparison, a 10p a litre swing in fuel prices, which feels dramatic, moves the 45 mpg car by roughly £130 a year. The car is worth seven times as much as the price move everyone talks about.

Which is not an argument for changing car, since depreciation on a newer one dwarfs the fuel saving. It is an argument for knowing your real figure before assuming fuel is a fixed cost you cannot influence.

Your real mpg is not the brochure figure

Official economy figures come from the WLTP test cycle, which is more realistic than the NEDC figures it replaced but is still a laboratory procedure. Real-world consumption is commonly 10 to 20% worse, and considerably worse than that for short urban trips from cold.

Use your own number instead. Fill to the brim, note the odometer, drive normally, fill to the brim again, then divide the miles covered by the litres it took and multiply by 4.54609. Two or three tanks gives a figure worth trusting.

Plug-in hybrids deserve particular scepticism. Their headline figures assume the battery starts full, so a PHEV that is never plugged in can return worse economy than the equivalent petrol car, because it is hauling a battery around for nothing.

Short journeys are the worst case for any petrol or diesel car. An engine below operating temperature can use roughly twice the fuel, which is why a five-mile commute costs far more per mile than the same five miles on a longer trip.

What actually reduces the bill

Speed is the biggest single lever. Aerodynamic drag rises with the square of speed, so 80 mph uses meaningfully more than 70 over the same distance and saves only a few minutes on a typical journey. Dropping to 60 on a long motorway run is the cheapest saving available.

Tyre pressure is the most neglected. Underinflated tyres increase rolling resistance, cost fuel continuously, and wear out sooner. Checking monthly takes two minutes.

Remove what you are carrying for no reason, especially roof bars and boxes, which hurt economy substantially at motorway speed even when empty. Air conditioning costs fuel at low speed; open windows cost more at high speed.

On price, supermarket forecourts are usually cheaper than motorway services by a wide margin, and a short detour rarely pays for itself unless you are filling a large tank. Our supermarket fuel comparison calculator works out whether a specific detour is worth it, and the Costco fuel calculator prices the membership against the saving.

Comparing petrol against electric, and against the train

Electric running costs depend enormously on where you charge. Home charging on an off-peak tariff can be a small fraction of petrol cost per mile; public rapid charging can approach it. Comparing on a single average is how people end up disappointed either way, so use the EV charging cost calculator with your actual tariff.

Against public transport, the honest comparison uses marginal cost rather than total. If you own the car regardless, the relevant figure for a single journey is fuel plus any parking and tolls, not a share of insurance and depreciation. If the journey is what justifies owning the car at all, the full cost applies.

For a shared journey, split the fuel. A 50-mile round trip at 14.1p a mile costs £7.07, which is well under a single rail fare on most routes once there are two people in the car, and considerably more than it once the car is empty.