So is it time to sell your diesel? For most owners of a post-2015 diesel covering ordinary mileage the answer is no, because the maths doesn't support a panic sale. Diesel averaged 199.9p a litre on 9 October, according to the RAC, yet a 50mpg diesel still costs about 18p a mile in fuel against nearly 20p for a 40mpg petrol, and used diesel values have already dropped 6.9% in a year, so selling now means taking that hit for a saving that barely exists. The exceptions are older diesels caught by clean air charges and cars that spend their lives on short town trips, and for those owners the sums look very different.
How much does diesel cost now?
Diesel passed £2 a litre for the first time on 2 October, when the RAC's average hit 200.01p and filling a 55-litre family car cost £110.01, which is £31.70 more than before the US-Iran war began on 28 February. It has since slipped back a fraction to 199.9p, with petrol at 174.79p, so diesel drivers are paying roughly 25p a litre more than their petrol neighbours.
The RAC reckons an average 45mpg diesel now costs about 20p a mile in fuel, or £2,020 a year if you cover 10,000 miles. That's a grim number, but it's only half the comparison, because whatever you replace your diesel with will need fuelling too.
Is a diesel still cheaper to run than a petrol?
Often it is, which surprises most people we put the numbers to. Diesel engines go further on a litre, and that extra economy cancels out most of the 25p price gap, as our sums below show.
| Car | Fuel Economy | Fuel Price | Fuel cost per mile | Fuel for 10,000 miles |
| Fugual diesel | 50mpg | 199.9p | 18.2p | £1,818 |
| Average diesel | 45mpg | 199.9p | 20.2p | £2,019 |
| Petrol equivalent | 40mpg | 174.79 | 19.9p | £1,987 |
| Petrol hybrid | 55mpg | 174.79 | 14.4p | £1,445 |
Regit calculation using RAC average pump prices on 9 October 2026 and 4.546 litres to the gallon. Swap in your own car's real-world mpg for a truer figure.
So swapping a decent diesel for the petrol version of the same car saves you next to nothing, and if your diesel is a frugal one it could cost you more. The swap that pays is a hybrid, which on these figures saves somewhere between £370 and £575 a year at 10,000 miles, or half that if you only do 5,000, and you'd need to weigh that against what you lose on the deal when you change cars.
What's happening to used diesel values?
Selling now means selling into a falling market. Solera cap hpi says three-year-old diesels with 30,000 miles have lost 6.9% of their value over the past 12 months, with the fall speeding up since March, and in October diesels slipped another 0.6% while hybrids rose 0.5% and electric cars 0.3%.
Buyers are cooling too. Diesels took an average of 41 days to sell in September, against 32 days for hybrids and 33 for EVs, and MarketCheck counted 90,495 used diesels sold that month, the lowest total of 2026 and well down on March's 103,288.
When everyone tries to sell the same kind of car at once, the people who sell first rarely get the best price. Fuel prices can also fall as quickly as they rise, and diesel dropped around 17p a litre in a single month in June when hopes of a peace deal grew, so a decision made at the very top of the market is one you might regret by spring.
Will ULEZ and Clean Air Zones cost you more?
For some diesel owners this matters far more than the pump price. London's ULEZ, which has covered all of Greater London since August 2023, charges £12.50 a day to diesel cars that don't meet the Euro 6 emissions standard, which in practice means most diesels built before September 2015.
Drive into the zone five days a week for 48 weeks a year and that's £3,000, more than the whole fuel bill in our table. Other UK cities run their own clean air and low emission zones with their own rules and charges, so if your daily drive crosses one, check where your car stands before you do anything else.
Could today's Budget change the sums?
Chancellor John Healey delivers his Autumn Budget today, 28 October, and reports suggest he'll freeze fuel duty at 52.95p a litre. If he doesn't, duty is due to rise by 3p in January 2027 and another 2p in March, which with VAT on top would add about £3.30 to every 55-litre fill-up of either fuel. Because the rise hits petrol and diesel equally it doesn't change which car is cheaper to run, only how much both of them cost you. For everything else in the Budget that affects drivers, see (LINK: What the Autumn Budget could cost you as a driver, https://www.regit.cars/car-news/what-the-autumn-budget-could-cost-you-as-a-driver).
When you should sell your diesel
Sell if your car is older than Euro 6 and you regularly drive into London or another city with a clean air charge, because the daily fee will cost you far more than any loss on the sale. The same goes if most of your driving is short trips around town, where a diesel's economy advantage shrinks and its diesel particulate filter is more likely to clog and need an expensive fix.
It also makes sense if you were planning to change your car within the next year anyway, or if a big repair bill is looming. In that case go for a hybrid, which is where the fuel savings are, and if you're weighing up an electric car instead, that explains why this winter's deals might not last.
When you should keep it
Keep it if it's a Euro 6 diesel that's reliable, mostly paid for and doing the job, especially if you cover high mileage on motorways or tow a caravan or trailer, which is exactly the work diesels were built for. Your fuel bill is higher than it was in February, but it's no worse than a petrol owner's, and replacing the car would mean selling at a low point and taking on the cost of a newer one.
Before you decide anything, get a free car valuation with Regit for your diesel, which takes a couple of minutes and gives you a far better starting point than a dealer's opening part-exchange offer.
Our verdict
If you've got a Euro 6 diesel and you don't pay a clean air charge, hang on to it, because at today's prices it costs no more per mile than the petrol you'd swap it for and selling now means taking a 7% hit on its value for nothing. Sell only if you're driving a pre-2015 diesel into a charging zone, doing short town trips, or changing cars soon anyway, and when you do, make the next one a hybrid, because that's the only swap that cuts your fuel bill.