Swapping to a more efficient car saves money only if the fuel saving outruns whatever the swap costs. This calculator puts both halves of that question in one place: the annual fuel saving at your real mileage and pump price, and the number of years the saving needs to repay the price difference. It deliberately ignores insurance and tax, which belong in a full running-cost comparison rather than a fuel one.
Both economy figures are miles per imperial gallon, converted to litres using the 4.54609 litres in a gallon before being multiplied by the pump price. Because mpg is a reciprocal measure, the saving depends on where on the scale you are. Over 10,000 miles, moving from 20 to 30 mpg saves 758 litres a year, while 50 to 60 mpg saves only 152.
At 10,000 miles a year, moving from 38 to 55 mpg cuts consumption from 1196 to 827 litres, saving £525.08 a year at £1.42 per litre. Against a £3,000 price premium that is a break-even of 5.7 years. The pump price, mileage and premium are all editable because all three vary enormously between drivers.
The formula
miles- Annual distance driven
mpg₁, mpg₂- Economy of the current and replacement vehicle, imperial
4.54609- Litres in one imperial gallon
premium- How much more the replacement costs to buy
How to use this calculator
- Enter your annual mileage and pick miles or kilometres.
- Enter the economy of the car you have and the one you are considering.
- Set the pump price and the fuel type for the carbon figure.
- Enter the extra purchase price to see the break-even period.
Worked examples
38 mpg to 55 mpg at 10,000 miles
The current car burns 1196 litres a year and the replacement 827, a saving of 370 litres or £525.08 at £1.42 per litre. A £3,000 premium takes 5.7 years to repay, and the carbon saving is 854 kg a year on petrol.
Where the same step is worth more
A driver covering 25,000 miles a year in a 22 mpg vehicle uses 5166 litres. Moving to 32 mpg drops that to 3552 litres and saves £2292 a year — far more than the same ten-mpg step higher up the scale.
How to read your score
Frequently asked questions
Why is the saving smaller at high mpg?
Because miles per gallon is a reciprocal. Going from 50 to 60 mpg over 10,000 miles saves 152 litres, whereas 20 to 30 mpg over the same distance saves 758. Improving the thirstiest vehicle in a household almost always yields the biggest absolute saving.
Should I include insurance, tax and servicing?
For a purchase decision, yes — but not here. This page isolates fuel so the effect is visible; standing costs belong in a cost-per-mile comparison where insurance, VED, servicing and depreciation sit alongside fuel.
Where do the carbon figures come from?
Burning a litre of petrol releases roughly 2.31 kg of CO2 and a litre of diesel about 2.68 kg, because diesel is denser and has more carbon per litre. These are tailpipe figures only and exclude the emissions of refining and delivering the fuel.
Does the break-even include depreciation?
No. It answers only how long the fuel saving takes to repay the extra purchase price. A more efficient car may also hold value differently, which can shorten or lengthen the real payback considerably.
What if both figures are US mpg?
The comparison still works, because the ratio between two US mpg figures is the same as between the equivalent imperial ones. The litre and pound totals will be wrong though, so convert to imperial first by multiplying by 1.2009.
Annual saving reference
| Replacement | From 25 mpg | From 30 mpg | From 38 mpg | From 45 mpg |
|---|---|---|---|---|
| 35 mpg | £738 | £307 | £0 | £0 |
| 40 mpg | £968 | £538 | £85 | £0 |
| 45 mpg | £1148 | £717 | £264 | £0 |
| 50 mpg | £1291 | £861 | £408 | £143 |
| 55 mpg | £1408 | £978 | £525 | £261 |
| 60 mpg | £1506 | £1076 | £623 | £359 |
| 70 mpg | £1660 | £1230 | £777 | £512 |
Zero means the replacement is no more efficient than the car it replaces. Scale linearly for a different annual mileage.