Cost Per Mile: Petrol vs Diesel at Today's UK Fuel Prices

Knowing your cost per mile makes it easy to compare cars, trips and fuel prices. Here is the formula and a ready-made table, using average prices from our database on 6 November 2026: petrol 169.56p and diesel 197.71p a litre.

The formula

Cost per mile (pence) = price per litre × 4.546 ÷ mpg

The 4.546 converts litres to UK gallons, because mpg is measured per gallon.

For example, a petrol car doing 40 mpg: 169.56 × 4.546 ÷ 40 = 19.3p a mile.

Cost per mile at today's prices

Read across: a diesel doing 50 mpg (18.0p a mile) is cheaper than a petrol doing 40 mpg (19.3p a mile), but a diesel doing 40 mpg (22.5p) is dearer.

What a year costs

At 8,000 miles a year:

• Petrol, 40 mpg: about £1,542
• Petrol, 50 mpg: about £1,233
• Diesel, 50 mpg: about £1,438
• Diesel, 60 mpg: about £1,198

Use your real mpg, not the brochure

Official mpg figures are tested under ideal conditions. For your own number, fill up fully, note the mileage, drive normally, then refill fully and note the litres added. Then:

mpg = miles driven ÷ (litres added ÷ 4.546)

Do this a few times and average it, since a single tank can be skewed by one long motorway trip or a cold week.

How to lower your cost per mile

Fuel price is only half the sum. The other half is how efficiently you use it. Small changes to how you drive and look after the car can make a real difference over a year. See 12 ways to cut your fuel bill.

What about hybrids and electric cars?

The same logic works for other fuels: take the cost of the energy and divide by the miles you get from it. We compare the options in petrol, diesel, hybrid or electric: running costs.

Is a diesel still worth it?

It depends on how much better its mpg is. At today's prices a diesel must be about 17% more efficient than a similar petrol to cost the same per mile. We explain in why diesel is dearer than petrol.

Tell our price search your car's mpg and it ranks stations by true cost, including the fuel burned getting there.