Petrol ↔ Diesel MPG Equivalence

Car adverts quote MPG figures, not cost per mile. This tool shows you what MPG a diesel car needs to achieve in order to cost the same per mile as a petrol car (or vice versa), based on current UK fuel prices.

Prices & Direction
Prices pre-filled from current UK national averages. Adjust to model different scenarios.
At these prices, a diesel needs to achieve 14.8% more MPG than its petrol equivalent to cost the same per mile.
Petrol MPG Pence per mile Equivalent Diesel MPG needed
20 36.0p 23.0
22 32.7p 25.3
24 30.0p 27.5
26 27.7p 29.8
28 25.7p 32.1
30 24.0p 34.4
32 22.5p 36.7
34 21.2p 39.0
36 20.0p 41.3
38 18.9p 43.6
40 18.0p 45.9
42 17.1p 48.2
44 16.3p 50.5
46 15.6p 52.8
48 15.0p 55.1
50 14.4p 57.4
52 13.8p 59.7
54 13.3p 62.0
56 12.8p 64.3
58 12.4p 66.6
60 12.0p 68.9
62 11.6p 71.2
64 11.2p 73.5
66 10.9p 75.8
68 10.6p 78.1
70 10.3p 80.4

Why MPG Alone Doesn't Tell You What a Car Costs to Run

Manufacturer brochures and used-car listings quote MPG figures, but the number you really care about — cost per mile — depends on the price of the fuel as well. A 50 MPG diesel and a 50 MPG petrol cover the same distance per gallon, but if diesel is 7p/litre more expensive, the diesel costs roughly 5% more per mile to drive.

This calculator does the conversion for you. Pick a direction (petrol → diesel or vice versa) and the calculator shows the MPG the alternative fuel would need to achieve to cost the same per mile, based on current UK pump prices.

When This Calculator Is Useful

  • Comparing two used cars — one petrol, one diesel — where the listings only show MPG.
  • Checking whether a "frugal" diesel really earns its higher purchase price at today's fuel prices.
  • Modelling fuel-price scenarios — e.g. "if diesel falls 10p, does the maths still favour petrol?"

Once you know the equivalent MPG, our Petrol vs Diesel Break-Even Calculator can tell you how many miles it takes for the fuel savings to recoup the diesel's higher purchase cost.

Frequently Asked Questions

MPG measures distance per gallon of fuel, but petrol and diesel cost different amounts per litre. A diesel car with 50 MPG and a petrol car with 50 MPG do not cost the same to drive — the one using the more expensive fuel costs more per mile.

It depends on current fuel prices. With petrol at 135p/litre and diesel at 142p/litre, a diesel needs roughly 5% more MPG — about 42 MPG — to match a 40 MPG petrol on cost per mile. The exact figure changes daily as pump prices move.

The calculator pre-fills the latest UK national average prices for unleaded (E10) and diesel (B7), sourced from the government's mandatory fuel price reporting scheme and updated every 30 minutes. You can override the values to model any scenario.