Fuel Cost Calculator
The number that makes a long commute feel real, and the one people forget when comparing two cars.
The calculation
If your car reports miles per gallon rather than litres per 100 km, convert first:
The two gallons are different — the imperial gallon is 4.546 litres and the US gallon 3.785 — so a car doing 40 mpg in the UK does about 33 mpg by US measurement. Comparing figures across the Atlantic without converting overstates American cars by 20 %.
Why real consumption exceeds the official figure
Manufacturer figures come from standardised laboratory cycles. Real-world consumption is typically 10–25 % worse, and the gap widens with:
- Speed. Aerodynamic drag rises with the square of speed, so power demand rises with the cube. Driving at 110 km/h instead of 130 can cut fuel use by 15–20 %.
- Short trips. A cold engine can use twice as much fuel in the first few kilometres. Very short journeys are disproportionately expensive.
- Stop-start traffic. Every acceleration converts fuel into kinetic energy that the brakes then discard as heat.
- Tyre pressure. Under-inflation by 20 % raises consumption by around 3 %.
- Roof boxes and racks. An empty roof rack can cost 10 % on a motorway journey.
- Air conditioning adds roughly 5 % in town, less at speed — where it beats open windows.
For budgeting, take the figure from your own fill-ups rather than the brochure: divide litres added by kilometres covered since the last fill, and multiply by 100.
Fuel is not the whole cost
Fuel is the visible cost of driving and usually not the largest. A fuller picture per kilometre includes depreciation, insurance, road tax, servicing, tyres, repairs and parking. Depreciation alone often exceeds fuel for a newer car.
A common planning rule is that total running costs are roughly two to three times the fuel cost. If a journey costs £8 in fuel, its true cost to you is closer to £20. That is the figure to use when comparing driving against rail, or when deciding whether a second car earns its keep.
For an electric vehicle the arithmetic is the same shape with different inputs: consumption in kWh/100km and price per kWh. Home charging at off-peak rates is typically a third to a quarter of the per-kilometre fuel cost of an equivalent petrol car; rapid public charging can be close to parity.
Frequently asked questions
Is it cheaper to drive or take the train?
For one person over a long distance, often the train once you include parking and the full running cost. For two or more people sharing a car, driving usually wins on cash cost. Compare the train fare against fuel plus parking plus a realistic allowance for wear — not fuel alone.
Does driving slower always save fuel?
Down to a point. Most cars are most efficient somewhere between 70 and 90 km/h. Below that, the engine runs inefficiently at low load and in a low gear; above it, aerodynamic drag dominates and rises steeply.
Do fuel additives improve economy?
Independent testing consistently finds little or no measurable benefit for mainstream additives in a well-maintained modern car. Correct tyre pressure, removing unnecessary weight and roof accessories, and smoother driving produce real, repeatable savings at no cost.