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Petrol vs Diesel Break-even Calculator

Estimate the fuel-only mileage needed for diesel fuel savings to recover a higher purchase price.

Use this calculator

Compare purchase prices and fuel-only running costs for petrol and diesel.

What your result means

In the example, diesel costs £2,000.00 more to buy but saves 3.8p per mile in fuel. The purchase premium is recovered after about 52,317 miles, or 4.4 years at 12,000 miles per year.

Worked example

Example values are illustrative and editable in the calculator.

Purchase-price difference
£2,000.00
Break-even mileage
52,317 miles

A break-even is shown only where diesel has both a higher purchase price and lower fuel cost per mile.

Formula

Break-even miles=Diesel purchase differencePetrol cost/mile−Diesel cost/mile\text{Break-even miles}=\frac{\text{Diesel purchase difference}}{\text{Petrol cost/mile}-\text{Diesel cost/mile}}

Fuel-only comparison

ScenarioEstimated result
Petrol cost per mile17p
Diesel cost per mile13.2p

Years to break even by annual mileage

Annual mileageApproximate years
6,0008.7
8,0006.5
10,0005.2
12,0004.4
15,0003.5
20,0002.6

What happens after break-even?

After the break-even mileage, diesel has a cumulative fuel-cost advantage on these assumptions. That does not mean it is cheaper overall, because other ownership costs are excluded.

If diesel costs more to buy but does not save fuel cost per mile, there is no fuel-cost break-even. A diesel that is not more expensive to buy has no purchase premium to recover.

Assumptions and limitations

  • This compares purchase-price difference and fuel costs only.
  • Insurance, servicing, repairs/maintenance, VED, AdBlue, depreciation, finance interest and resale value are excluded.

Helpful questions

Is diesel always cheaper at high mileage?

No. A break-even requires diesel to save fuel cost per mile under the entered assumptions.

Should I use real-world MPG?

Yes. Real-world MPG makes the fuel-cost comparison more representative.

What if the diesel is cheaper to buy?

There is no diesel purchase premium to recover.

Does this include VED and servicing?

No. It is a fuel-cost purchase-price break-even calculation only.

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