Enter your distance, real-world MPG and the price you paid at the pump. Uses UK imperial gallons (4.54609 litres).
Diesel is sold in litres in the UK but economy is usually quoted in miles per gallon — and that is an imperial gallon, 4.54609 litres, not the smaller US gallon. The maths the calculator runs is:
At 38 MPG and 151.9p a litre, a 100-mile run uses about 12 litres and costs roughly £18. Drop to 30 MPG — a loaded van in traffic — and the same trip costs about £23. That is the single biggest lever on your fuel bill: real-world MPG, not the pump price.
| Vehicle | Real-world MPG | Cost per 100 miles* |
|---|---|---|
| Small hatchback (1.5 dCi, Corsa, Fiesta) | 55–65 | £10.60–£12.50 |
| Family saloon / estate (Passat, 3 Series) | 45–55 | £12.50–£15.40 |
| SUV (Tiguan, X3, Qashqai) | 38–45 | £15.40–£18.20 |
| Medium van (Transit Custom, Transporter) | 30–36 | £19.20–£23.00 |
| Large van / Luton (Sprinter, Boxer) | 24–29 | £23.80–£28.80 |
*At 151.9p per litre. Change the price in the calculator above for today's figure.
A modern diesel typically returns 10–15% better MPG than the equivalent petrol engine, but diesel usually costs more per litre. When the gap at the pump is small, the diesel wins on cost per mile; when diesel is 8–10p a litre dearer, the advantage mostly disappears on short journeys where the engine never warms up properly.
Diesel also carries extra costs that MPG comparisons ignore: DPF regens on short runs, AdBlue, and generally higher servicing. If most of your miles are urban and under 10 miles, a petrol or hybrid usually works out cheaper overall.
To get a true figure, fill the tank, reset the trip, drive normally, refill and divide miles by the litres you put in — then multiply by 4.54609. That number, not the brochure figure, is what this calculator should use.
If you run a van or car for work, you can either claim the actual fuel cost (keep every receipt and a mileage log) or use HMRC's simplified mileage rates — 45p per mile for the first 10,000 miles and 25p thereafter for cars and vans. Simplified rates are easier but usually pay less than actual cost for a thirsty vehicle; the calculator above gives you the actual figure to compare.
The main calculator handles petrol, diesel and EV side by side, with cost per mile and CO₂.
Divide the price per litre by the MPG, then multiply by 4.54609 and by 100. At 38 MPG and 151.9p a litre that is about 18.2p per mile.
UK imperial gallons — 4.54609 litres. A US gallon is 3.785 litres, so US calculators return a noticeably different number.
Official WLTP figures come from a laboratory cycle. Cold starts, town driving, load, air conditioning and motorway speeds all pull real-world MPG down by 10–20%.