Fuel economy comparison
When choosing between two cars, the sticker economy figure is abstract — what matters is the money you spend at the pump over a year. This tool turns each vehicle’s fuel economy into an annual running cost and shows the difference, so you can see exactly how much a more efficient car saves over your real mileage.
How it works
Pick a unit system, enter your annual distance and fuel price, then each vehicle’s economy. The tool computes yearly fuel cost for each car and subtracts to find the saving:
- MPG mode (miles, price per gallon): gallons = distance ÷ MPG, then cost = gallons × price.
- L/100km mode (km, price per litre): litres = (consumption ÷ 100) × distance, then cost = litres × price.
Because higher MPG and lower L/100km both mean better economy, the tool always identifies the cheaper car and the annual saving.
Worked example
Driving 12,000 miles a year at £6 per gallon, comparing a 30 mpg car with a 45 mpg car:
- 30 mpg: 12,000 ÷ 30 = 400 gallons × £6 = £2,400/year
- 45 mpg: 12,000 ÷ 45 ≈ 266.7 gallons × £6 ≈ £1,600/year
- annual saving ≈ £800
| Vehicle | Economy | Gallons/year | Annual cost |
|---|---|---|---|
| A | 30 mpg | 400 | £2,400 |
| B | 45 mpg | 266.7 | £1,600 |
| Saving | — | — | £800 |
Over five years that £800/year difference becomes £4,000 in fuel savings — more than enough to factor into a car purchase decision.
What the comparison does not cover
This tool isolates fuel cost only. A full running-cost comparison should also account for:
- Road tax (VED): In the UK, low-emission vehicles pay less VED, and some EVs or hybrids are exempt entirely.
- Insurance: Premiums vary significantly between models and cannot be modelled from economy alone.
- Servicing and tyres: Diesel engines are generally more expensive to service than petrol. EVs have fewer moving parts but higher tyre wear from battery weight.
- Purchase price premium: A more efficient car often costs more to buy. Compare the fuel saving against the price difference to find the real payback period — for example, if the efficient car costs £3,000 more and saves £800/year, the fuel payback is about 3.75 years.
Tips for accurate results
- Use real-world economy figures rather than manufacturer WLTP ratings. Official figures are measured under lab conditions and real-world consumption is typically 10–20% higher for petrol and diesel cars.
- If your annual mileage is uneven — a long commute in winter but lighter use in summer — use the annual total rather than trying to weight it.
- For petrol/diesel comparisons, use the price of whichever fuel the car takes. For a petrol vs diesel comparison where fuel prices differ, run each car against its own price and compare the totals.
All maths happens in your browser — your data never leaves your device.