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Diesel's Slow Slide Is the Used Market's Loudest Signal
Solera cap hpi's September read shows hybrids and electric cars firming while petrol and diesel soften at three years old. The used market is repricing fuel types.
Used car values move in small numbers. A monthly change of half a percent barely registers on its own, which is why the fuel-type split inside the latest figures is worth more attention than the headline. Solera cap hpi's September read of the market shows hybrid values up 0.5% and electric up 0.3% month to date at three years old, while petrol slipped 0.5%, diesel fell 0.6% and plug-in hybrids dropped furthest at 0.8%. The market is quietly repricing fuel types, and the direction of travel says a lot about where drivers think the next few years are going.
The overall market is soft
Start with the backdrop. The average value of a three-year-old used car fell 0.3% so far in September, following a 0.9% decline in August. Values were also down for one-year-old cars by 0.2%, five-year-olds by 0.8% and ten-year-olds by 2%. Nothing dramatic, but the drift is downwards across every age band.
Against that soft background, the fuel-type split stands out. Hybrids and battery electric cars are the only categories moving up, and they are doing it while the overall market falls. Solera cap hpi's head of current valuations, Chris Plumb, noted that hybrids had a good month and that BEVs "continue to settle down" after several years of sharp value movements. In his words, higher petrol and diesel prices "only make that comparison more interesting".
Diesel's decline is the tell
The diesel number is small, 0.6% down at three years old, but its context is not. The average value of a three-year-old diesel car has now fallen for five consecutive months, the longest unbroken slide of any fuel type in the current data.
The cause is visible at every filling station. Diesel prices have been climbing towards record territory through September, and Solera cap hpi attributes the used-value decline directly to high pump prices. The logic is straightforward: a driver shopping a three-year-old diesel can see what it will cost to run, and dealers know it too. As Plumb put it, "Diesel at around 195p a litre is difficult to ignore", particularly against petrol, so dealers are being more selective about which diesel stock they buy.
That is a practical warning for anyone selling a diesel. The car itself has not changed, but the running-cost comparison has moved against it, and trade buyers price in what the next owner will feel every week. If your diesel is due for a change, the sooner you sell, the better.
Electric values are finding their floor
The electric story is the quiet one, and arguably the more useful. Used BEV values rose 0.3% in September, which follows a 0.1% rise in August, when they were the only fuel type to gain while the wider market dropped 0.9%.
Two things make that interesting. First, supply is surging: fleet and rental returns are pushing used-EV volumes up sharply, with many more electric cars coming back to the market than a year ago. Basic economics says that much extra supply should push prices down. Plumb's observation was that buyers are absorbing the extra stock, with no corresponding fall in values, and that clean cars at the right price are still selling.
Second, the price advantage has shifted to the buyer's side. Electric values spent several years correcting hard from their 2022 peaks, and that correction has done its work. Combined with the AA's finding that used EVs now cost just 1% more than petrol equivalents, the used market is close to the point where the fuel-type premium disappears entirely. Our used EV gap analysis tracks that number, and the September data suggests the gap is still closing. With values firming while supply grows, the repricing that began earlier this year looks like a trend rather than a blip.
The plug-in hybrid number deserves its own word of caution. PHEVs fell 0.8%, the steepest drop of any fuel type, which is awkward for a technology often sold as the best of both worlds. A used plug-in hybrid's value now depends heavily on whether its next owner will actually charge it. Buyers who treat it as a petrol car with occasional electric trips pay petrol-level fuel bills on a complicated drivetrain, and the trade is starting to price that uncertainty in.
What it means if you're buying or selling
For sellers, the fuel-type hierarchy in the current data is clear: hybrids and electric cars are holding value best while petrol and diesel drift. For buyers, the same data read the other way round says used electric cars are approaching the point where they cost no more than the equivalent petrol car while costing far less to run, and the mechanical worry that used to justify a discount has shrunk, as the recent MOT study of high-mileage EVs showed.
One caution on the numbers: these are month-to-date movements at the three-year benchmark, not a full-month final figure, and three-year-old values at 60,000 miles are the trade's yardstick rather than a promise about any specific car. Condition, mileage and spec move any individual car a long way from the average. But direction matters more than a decimal point, and September's direction is unmistakable: the used market is repricing what fuel a car burns, and diesel's slide is the clearest signal of where it expects this to end.