How to Calculate Fuel Cost for a Journey (With Worked Examples)
Journey fuel cost is miles divided by mpg, times 4.54609 litres per gallon, times the price per litre. A 100-mile trip at 40 mpg uses 11.4 litres and costs £19.78 in petrol or £22.64 in diesel, at DESNZ prices for the week of 26 October 2026.
What is the formula for journey fuel cost?
Divide the miles by your mpg to get gallons, multiply by 4.54609 to get litres, then multiply by the price per litre. Written out: cost = (miles / mpg) x 4.54609 x price per litre.
Use the price in pounds, so 174.07p becomes £1.7407. If you want pence per mile instead, drop the miles from the formula: pence per mile = price in pence per litre x 4.54609 / mpg.
The 4.54609 is the number of litres in one UK gallon. Your mpg must be a UK (imperial) figure for this to work. If your source is American, convert it first with our guide to UK vs US mpg.
What did a petrol and diesel journey cost this week?
At the Department for Energy Security and Net Zero (DESNZ) weekly road fuel prices for the week of 26 October 2026, petrol was 174.07p a litre and diesel was 199.22p. Here is a 100-mile journey in a car that does 40 mpg:
- Gallons: 100 / 40 = 2.5
- Litres: 2.5 x 4.54609 = 11.37 litres
- Petrol cost: 11.37 x £1.7407 = £19.78
- Diesel cost: 11.37 x £1.9922 = £22.64
That is 19.8p a mile in petrol and 22.6p a mile in diesel. These are fuel costs only. Tyres, servicing, depreciation and insurance are extra.
How much does a typical UK trip cost?
The table below applies the formula at the same week’s prices to four common trip lengths, at 40 mpg and 55 mpg. Figures are rounded to the nearest penny.
| Trip | Litres at 40 mpg | Petrol at 40 mpg | Diesel at 40 mpg | Litres at 55 mpg | Petrol at 55 mpg | Diesel at 55 mpg |
|---|---|---|---|---|---|---|
| 50 miles | 5.7 | £9.89 | £11.32 | 4.1 | £7.19 | £8.23 |
| 100 miles | 11.4 | £19.78 | £22.64 | 8.3 | £14.39 | £16.47 |
| 200 miles | 22.7 | £39.57 | £45.28 | 16.5 | £28.78 | £32.93 |
| 300 miles | 34.1 | £59.35 | £67.93 | 24.8 | £43.16 | £49.40 |
Going from 40 mpg to 55 mpg cuts the 300-mile petrol bill by £16.19. The vehicle matters, and so does how you drive it.
How much fuel will I use?
Litres used = miles / mpg x 4.54609. A 250-mile trip at 45 mpg is 250 / 45 = 5.56 gallons, which is 25.3 litres. Check your tank or tank range against that number before you set off.
If you do not know your mpg, you can work it out from a full tank to full tank. Our guide on how to calculate mpg shows the method. The dashboard figure is a starting point, but your own measured figure is better.
How do you split fuel costs between passengers?
Divide the total fuel cost by the number of people in the vehicle, including the driver. A 200-mile petrol trip at 40 mpg costs £39.57 in fuel. Shared between four people, that is £9.89 each.
Two points keep it fair. First, share the fuel cost of the whole journey, not the cost of extra miles to pick people up, unless you agree otherwise in advance. Second, agree the mpg and price beforehand, or use the pump receipt, so nobody argues about the sums afterwards. Fuel is only part of the cost of running a car, so many car-sharers also add a small amount for wear.
Does speed change the cost of a journey?
Yes. The faster you drive on fast roads, the more fuel each mile uses. The Department for Transport’s fuel consumption curves in its TAG data book (v2.03, May 2026, table A1.3.8) give fuel per mile at each average speed. This is how much less fuel per mile you use at the lower speed:
| Vehicle | 70 to 60 mph | 68 to 60 mph | 68 to 56 mph |
|---|---|---|---|
| Diesel van | not covered | 14.5% | 20.0% |
| Petrol van | 16.1% | 12.6% | 16.4% |
| Diesel car | 9.6% | 7.5% | 10.1% |
| Petrol car | 8.9% | 6.8% | 8.8% |
DfT’s diesel van curve stops at 68 mph, so we do not quote a diesel van figure above it. A petrol car doing 100 miles at 60 mph instead of 70 mph uses 8.9% less fuel. On the £19.78 journey above, that is a saving of about £1.76, assuming the 40 mpg figure was measured at 70 mph.
For a fleet, small savings per trip multiply fast. See the speed and fuel calculator for the full-fleet version.
How does a fleet work out fuel cost per journey?
A fleet uses the same formula, but with each vehicle’s own measured mpg and the price it actually pays. Cards, bunkering and bulk contracts can differ from the pump price in the DESNZ series. If the fleet runs a mix of vehicles, calculate each separately.
The fuel cost calculator does the journey sums for you, pre-filled with this week’s DESNZ prices. For the wider picture, including tyres, servicing and depreciation, read how to calculate fleet cost per mile.
Frequently Asked Questions
What is the formula for fuel cost per mile?
Pence per mile = price per litre in pence x 4.54609 / mpg. At 174.07p a litre petrol and 40 mpg that is 19.8p a mile. At 199.22p a litre diesel and 40 mpg it is 22.6p a mile. Both use DESNZ prices for the week of 26 October 2026.
How do I work out how much fuel I will need for a trip?
Divide the miles by your mpg, then multiply by 4.54609 to get litres. For 250 miles at 45 mpg that is 25.3 litres. Add a margin for traffic, hills and loaded vehicles, because your real mpg on the day may be lower than your average.
How do I convert mpg to cost per mile?
Multiply the price per litre by 4.54609 and divide by your mpg. The result is in the same unit as the price, so use pence for pence per mile. It is the quickest way to compare two vehicles on running cost.
Is it cheaper to drive at 60 mph than 70 mph?
Yes. The Department for Transport’s curves show a petrol car using 8.9% less fuel per mile at 60 mph than at 70 mph, and a petrol van 16.1% less. The journey takes longer, so weigh the fuel saving against driver time.
Should I split fuel costs by miles or by people?
For a shared car trip, splitting by the number of people is simplest. If passengers join or leave part way, split each leg by the people in the car for that leg. Use the actual fuel cost for the journey.
Cut your fleet’s fuel bill
Find your own number with the fuel cost calculator. Enter your vehicle’s mpg and your journey length, and it works out litres and pounds using this week’s DESNZ prices. For fleet-wide savings from slower speeds, use the speed and fuel calculator.
A speed cap saves fuel on every journey without the driver having to remember it. The Department for Transport’s fuel consumption curves show a diesel van using 14.5% less fuel per mile at 60 mph than at 68 mph. How much of that reaches your bill depends on how many of your miles are driven above the cap.
Talk to us. AutoKontrol has made speed limiters in Britain since 1985, with nationwide installation where we come to you. Book a free consultation or get a quote.
Related Products
DBW Speed Limiter for Cars, Vans, HGVs and Buses 
GeoKontrol 2 GPS Speed Limiter with Geofenced Zones 
SENT Speed Limiter for Euro VI Trucks and Vans 
AutoKontrol Technical Team
World leaders in speed limiter technology with 41+ years of experience. Trusted by fleet operators, logistics companies, and vehicle manufacturers worldwide.
ISO 9001 • UTAC • TÜV • VCA Approved • British manufactured since 1985