The formula behind the fuel gauge
Annual fuel cost equals annual distance, multiplied by consumption divided by 100, multiplied by the price per litre. Drive 12,000 km a year in a car that uses 6.5 litres per 100 km, with fuel at £1.50 a litre, and you burn 780 litres and spend £1,170. Those numbers are illustrative: swap in your own.
The formula is simple; accuracy lives in the inputs. Take distance from your own records, such as two MOT certificates, service invoices or a note of the odometer on the same date each year. Take consumption from your own fill-ups: brim the tank, reset the trip meter, and divide litres added at the next fill by kilometres driven, times 100.
Converting mpg to litres per 100 km
UK drivers often think in miles per gallon. To convert UK mpg to L/100 km, divide 282.48 by the mpg figure: 40 mpg is roughly 7.1 L/100 km, and 50 mpg is roughly 5.65 L/100 km. Remember that mpg is "higher is better" while L/100 km is "lower is better". US gallons are smaller than UK imperial gallons, so US mpg figures are not directly comparable with UK ones.
Why your real consumption differs from the official figure
Official figures come from standardised laboratory test cycles. In the UK and EU the current procedure is WLTP, which replaced the older NEDC test and was designed to be more representative. Even so, its job is to make cars comparable with each other, not to predict your bill. Common reasons real-world consumption runs higher:
- Short journeys and cold starts, before the engine reaches operating temperature.
- Sustained high motorway speeds, where aerodynamic drag rises steeply.
- Under-inflated tyres, which increase rolling resistance.
- Roof boxes, bike racks and heavy loads left on or in the car.
- Air conditioning, heated screens and seat heaters.
- Hard acceleration, late braking and congested stop-start traffic.
Comparing powertrains
| Powertrain | What you measure | Strengths | Watch-outs |
|---|---|---|---|
| Petrol | L/100 km, pump price | Simple, suits mixed and short-trip use | Consumption climbs on long high-speed runs |
| Diesel | L/100 km, pump price | Efficient on long motorway journeys | Particulate filters may need long runs; many need AdBlue top-ups |
| Full hybrid | L/100 km, pump price | Strong in town, no plug needed | Smaller advantage at steady motorway speeds |
| Plug-in hybrid | L/100 km and kWh/100 km | Low cost if charged regularly | Costly if rarely charged, as it carries a heavy battery |
| Electric | kWh/100 km, price per kWh | Low energy cost with home charging | Public rapid charging can cost far more than a home tariff |
Using the calculator for an electric car
Swap litres for kilowatt-hours: annual cost equals distance, times kWh per 100 km divided by 100, times price per kWh. If you charge partly at home and partly in public, work out a blended price weighted by how much energy comes from each. Allow for charging losses too, since energy drawn from the wall is a little higher than what reaches the battery.
Cost per kilometre: the number that ties it together
Divide any annual cost by annual distance and you get a per-kilometre figure. Fuel per km tells you what a trip costs at the pump. Total cost per km, covering fuel, insurance, tax, servicing, interest and depreciation, tells you what the car costs to use. High-mileage drivers spread fixed costs over more kilometres, so their total per-km figure falls even as fuel spend rises; low-mileage drivers often find fixed costs dominate.
Pros of thinking per km
- Compares cars of different price and size fairly.
- Helps weigh driving against trains, buses or car clubs.
- Useful for checking business mileage reimbursement.
Cons
- Hides that many costs are fixed.
- Changes whenever your mileage changes.
- It is not a cash-flow figure for budgeting.
Practical ways to lower fuel spend
Check tyre pressures monthly against the figure in your handbook. Remove roof racks when unused. Combine short errands into one trip. Anticipate traffic to avoid unnecessary braking. Then feed your improved consumption into the full ownership calculator to see the effect on your annual total.