How the journey cost is calculated
litres = (miles ÷ mpg) × 4.54609
cost (£) = litres × pence per litre ÷ 100
cost (£) = litres × pence per litre ÷ 100
One imperial gallon is exactly 4.54609 litres. A US gallon is only 3.785 litres, which is why US calculators give the wrong answer for UK mpg figures — typically understating fuel used by about 17%.
Worked example
Driving 200 miles at 45 mpg uses 200 ÷ 45 = 4.44 imperial gallons, or 20.2 litres. At 140p per litre that's 20.2 × £1.40 = £28.29. A return journey would be £56.57.
Which mpg figure should I use?
- Official figures for newer cars come from the WLTP test. They're more realistic than the older NEDC figures, but motorway speeds and short cold trips can still give lower real-world numbers.
- Many cars show average mpg on the dashboard — that's usually the most useful figure for your own driving.
- Diesel cars often have higher mpg but diesel usually costs more per litre, so compare total cost rather than mpg alone.
Costs this doesn't include
Journeys into some city centres can carry clean-air zone, congestion or toll-road charges, and parking can easily cost more than the fuel. Add those separately, or use the drive vs fly calculator to compare against flying or another option.
Splitting the cost with passengers? Set the number of people above, or track a whole trip with the expense splitter.