Prices checked at 6:42pm BST on 5 October 2026.
A diesel car may travel further on each gallon, but that only saves money on fuel if the extra miles outweigh the higher pump price. At current prices, the answer is roughly 14½% better MPG.
I compared petrol and standard-diesel prices at the same forecourts in Fuel Finder's data on 5 October 2026. Among 6,299 qualifying sites reporting both fuels, the median diesel price required 14.46% more miles per gallon to match petrol's fuel cost per mile. At 6,261 of those sites, or 99.4% of this sample, the required advantage exceeded 10%.
That is a comparison of fuel costs for two comparable vehicles. It does not include purchase price, servicing, insurance, tax or the cost of changing cars.
What does 14½% mean on the road?
If a petrol version returns 45mpg, a comparable diesel version needs to return about 51.5mpg to break even on fuel at the sample's average prices: 174.95p per litre for petrol and 200.22p for diesel.
Over 10,000 miles at those prices:
| Petrol car | Diesel car | Diesel fuel-cost difference |
|---|---|---|
| 45mpg | 50mpg | About £53 more |
| 45mpg | 51.5mpg | Approximately break-even |
| 45mpg | 55mpg | About £112 less |
These are illustrations, not claims about any particular model. Your real-world MPG, mileage and local pump prices determine your own result. The calculation is simple: diesel break-even MPG = petrol MPG × diesel price ÷ petrol price.
The hurdle has risen
I also checked forecourts with usable petrol and diesel price records both now and at the close of 5 September. This fixed group contained 5,657 sites, avoiding a comparison distorted by different stations appearing in each sample.
A month ago, their average diesel break-even requirement was 13.15% more MPG. It is now 14.46% for those same sites, a rise of 1.31 percentage points. The requirement increased at 4,625 of them, or 81.8%.
The change reflects a widening petrol–diesel price gap in this sample. Its average gap rose from 21.60p to 25.24p per litre. Fuel Finder's separately sampled UK Fuel Price Index also shows diesel rising faster than petrol over the last 30 days, though its headline averages should not be substituted for a same-forecourt comparison.
Your local threshold may differ
The national figure is a guide, not a price available at every pump. In the 275 qualifying forecourts in the BT postcode area, the average break-even requirement was 16.21% more MPG. Petrol there averaged less than in the rest of the qualifying sample, but diesel carried a larger relative premium. A lower absolute diesel price does not necessarily make diesel better value against local petrol.
The practical test is to compare the two prices you would actually pay, then use realistic MPG for the vehicles you are weighing up. If you already own a diesel, this calculation cannot turn it into a petrol car; comparing nearby diesel prices remains the more immediate way to reduce a fill's cost. As my earlier local-price study found, the availability of a meaningfully cheaper nearby forecourt varies by location.
How I measured it
This analysis used Fuel Finder's stored, time-stamped forecourt prices on the evening of 5 October 2026. The current comparison includes active, open sites with valid locations and accepted E10 petrol and standard B7 diesel source observations no more than 14 days old. Each forecourt has one vote. Invalid prices and test records were excluded; motorway and membership forecourts were retained. The month-earlier comparison uses the same sites where both fuels also had usable records at the earlier date.
The results describe reported forecourt prices, not transactions, fuel volumes or the number of affected drivers. A price record can be older than the day of this analysis if the reported price has not changed. Prices and the qualifying sample will change as new reports arrive. The government's Fuel Finder scheme supplies time-stamped retail price data; check the displayed reporting time and pump price before filling.