For years the advice was easy to remember: buy diesel if you cover a lot of miles, buy petrol if you mostly potter about town. The maths behind that rule still holds, but the gap has narrowed and a few costs that buyers used to ignore now swing the answer. This guide works through the pump price, the fuel economy difference and the running costs that sit either side of the fuel gauge, so you can see roughly where the line falls for the way you actually drive.
The short answer. A diesel typically returns 15 to 25 percent more miles per gallon than the equivalent petrol car, which is its main draw. Against that, diesel usually costs a few pence more per litre, diesel cars often cost more to buy, and some of them cost more to keep on the road. As a rough guide, diesel starts to repay the difference somewhere above 12,000 to 15,000 miles a year, mostly on longer motorway trips. Below that, or for mainly short urban runs, petrol tends to be the cheaper and simpler choice. Fuel prices used here are illustrative examples; look up today's pump prices for your own figure.
The two numbers that decide it
Strip away everything else and the fuel cost of any car comes down to two figures: what you pay per litre, and how many miles the car does on a gallon. A gallon is 4.546 litres, so the cost of driving a mile is the price per litre multiplied by 4.546, then divided by the miles per gallon.
Diesel pulls those two numbers in opposite directions. It usually sits a few pence dearer per litre at the pump, which pushes the cost per mile up. But a diesel engine squeezes more miles out of each litre, which pulls the cost per mile back down. Whether diesel ends up cheaper per mile depends on whether its economy advantage is big enough to beat its price premium at the pump, and that varies a lot with the sort of driving you do.
You can run your own version of this sum with the petrol cost per mile calculator, which takes your pump price and your real miles per gallon and gives a pence-per-mile figure for either fuel.
Why diesel goes further per litre
A litre of diesel carries a little more energy than a litre of petrol, and diesel engines are built to use it more effectively. They run at higher compression and burn a leaner mixture, which is why a diesel of the same size and power as a petrol car will often return noticeably better economy, especially at a steady cruise.
The advantage is largest on the motorway, where the engine settles into a constant, efficient load and the tall gearing lets it turn slowly. It shrinks around town. A diesel takes longer to warm up, dislikes short stop-start journeys, and needs its exhaust hot enough to keep the emissions kit working properly, none of which suits a two-mile school run. So the same diesel that returns 60 miles per gallon on a long trip might drop to the low forties on cold urban hops, while a small petrol car often holds up better in traffic. The economy figure on the brochure is a best case; your own number depends heavily on your journeys.
Cost per mile: a worked example
Take an example petrol price of 140p a litre and an example diesel price of 148p a litre, with each car returning economy typical for its type. The table shows how the pence per mile lands. These are illustrative numbers; your pump prices and your real economy will differ.
| Car and typical use | Fuel and price | Economy | Cost per mile |
|---|---|---|---|
| Small petrol car, mixed driving | Petrol, 140p/litre | 55 mpg | ~11.6p |
| Family petrol car, mixed driving | Petrol, 140p/litre | 45 mpg | ~14.1p |
| Family diesel, mostly motorway | Diesel, 148p/litre | 55 mpg | ~12.2p |
| Family diesel, mostly short town trips | Diesel, 148p/litre | 45 mpg | ~15.0p |
| Diesel estate, long steady runs | Diesel, 148p/litre | 60 mpg | ~11.2p |
Cost per mile calculated as price per litre times 4.546, divided by miles per gallon. Fuel prices and economy are illustrative examples.
Two things stand out. When the diesel achieves the economy it is built for, around 55 to 60 mpg on longer runs, it comes in a couple of pence a mile below a 45 mpg petrol car despite the dearer fuel. But a diesel worked hard around town, dropping to 45 mpg, actually costs more per mile than a frugal small petrol car, because it is paying the diesel premium at the pump without earning the economy back. The fuel type does not decide the winner on its own; the type of driving does.
The mileage where diesel starts to pay
The per-mile fuel saving from diesel is usually small, on the order of 2p in the examples above. That saving has to repay the extra you spend to own a diesel in the first place, and for a new car that premium can run to four figures. Divide the extra purchase cost by the per-mile saving and you get the distance you must drive before the diesel breaks even on fuel alone.
Say a diesel costs £1,500 more to buy than the petrol version and saves 1.9p a mile on fuel. That is £1,500 divided by 1.9p, roughly 79,000 miles before the fuel saving has clawed back the price difference. Spread over a typical five or six year ownership, that only works if you are covering something like 13,000 to 16,000 miles a year. At 8,000 miles a year the diesel might never catch up before you sell it; at 25,000 miles a year it pays for itself comfortably and keeps saving after that.
The sum shifts on the used market, where the price gap between an equivalent petrol and diesel is often much smaller, sometimes only a few hundred pounds. A smaller premium means a shorter break-even distance, which is one reason a high-mileage buyer looking at used cars can still find diesel makes sense. Work it out with your own numbers rather than the old rule of thumb, because the rule assumed a fuel-price gap and a purchase premium that are not fixed.
Beyond the pump: servicing, AdBlue and DPFs
Fuel is not the whole running cost, and this is where diesel can quietly give some of its saving back. Modern diesels carry more emissions hardware than petrol cars, and that hardware needs the right sort of use to stay healthy.
The diesel particulate filter, or DPF, traps soot and periodically burns it off in a process that needs a good stretch of hot, fast running to complete. A car used only for short cold trips may never get hot enough, the filter clogs, and clearing or replacing it can be an expensive job. Many newer diesels also use AdBlue, a fluid dosed into the exhaust to cut nitrogen oxides, which you top up every few thousand miles at a modest cost. Turbochargers, high-pressure injectors and dual-mass flywheels are all common on diesels and can be pricey if they fail. None of this is a reason to avoid diesel if your driving suits it, but a petrol car generally has fewer of these parts to look after, and its servicing can work out a little cheaper.
Tax, clean air zones and resale
A few costs sit outside the fuel and servicing that can matter more than the pence per mile. First-year road tax has at times been set higher for diesels that do not meet the latest lab test standards, so check the vehicle excise duty band for the specific model before you buy. Older diesels also face charges to enter some city clean air and ultra low emission zones, where a pre-Euro 6 diesel pays a daily fee that a newer car avoids; the equivalent cut-off for petrol is an older Euro 4 standard, so a diesel typically needs to be more recent to drive in free. If you regularly drive into a charging zone, that fee can dwarf any fuel saving.
Resale is the other line to watch. Demand for diesel has softened as cities tighten emissions rules and buyers look to electric, which has taken some of the shine off used diesel values. A car that holds its value less well costs you more overall even if the fuel is cheap, so factor depreciation into the comparison rather than looking at the pump alone.
Cutting the cost whichever you drive
Once you have chosen a car, the biggest lever on its running cost is how you drive it. Smooth, anticipatory driving, keeping to sensible speeds, correct tyre pressures and not lugging around a bootful of dead weight all trim the fuel bill on petrol and diesel alike, and they are gathered in the hypermiling guide. Running the air conditioning adds a little to fuel use, which the keeping your car cool guide puts into perspective, and it is rarely worth sweating over.
It is also worth widening the question. For a lot of drivers the cheaper-per-mile answer is no longer either petrol or diesel but electric, at least where charging at home is possible. The electric car versus petrol running cost guide sets out that per-mile gap, and EV cost versus petrol per year scales it across a year's mileage, so you can see how all three stack up before committing to a fuel at all.
The verdict
Diesel remains the cheaper fuel per mile for the driving it was designed for: high annual mileage made up largely of longer, steady journeys, where the economy advantage is real and the emissions kit stays healthy. If you cover 15,000 miles a year or more on the motorway, a diesel still tends to win once everything is added up, and more so on the used market where the purchase premium is small.
For lower mileages, mainly short urban trips, or drives into a clean air zone, petrol is usually the better bet. It costs less to buy, carries fewer of the parts that make diesels expensive to maintain, and is not paying a fuel premium it cannot earn back. Run the numbers with your own pump prices and your real economy in the cost per mile calculator, weigh in tax and depreciation, and check whether an electric car would undercut both before you decide. The wider driving and fuel section pulls the rest of the picture together.