Diesel vs Petrol

Fuel Efficiency: Where Diesel Still Wins

Diesel engines are fundamentally more thermally efficient than petrol engines — they extract more energy from each litre of fuel. In real-world UK driving, a diesel version of a given car will typically return 15–20% more MPG than the petrol equivalent.

This advantage is most pronounced on long motorway journeys, where diesel engines run at a steady speed and temperature. In stop-start city traffic, the gap narrows significantly — and hybrids often outperform both.

You can check how your current car's MPG stacks up using our guide to what is a good MPG, or measure it precisely with the MPG calculator.

The DPF Problem: Diesel's Achilles Heel

Every modern diesel car is fitted with a diesel particulate filter (DPF) — a device that traps soot from the exhaust. The DPF cleans itself by reaching high temperatures, which typically happens on longer journeys at motorway speeds.

If you mainly do short trips (under 10 miles), the DPF never gets hot enough to regenerate properly. Over time it becomes blocked — and a DPF replacement can cost £1,000–£3,500 depending on the car. This is arguably the biggest practical argument against diesel for urban drivers.

Petrol engines have no equivalent issue. They're equally happy on short or long journeys.

Road Tax (VED)

Since April 2017, diesel cars that don't meet the stricter RDE2 emissions standard pay a one-band supplement in their first year of vehicle excise duty. From year two onwards, most cars (petrol or diesel) pay the same flat standard rate — currently £190/year for cars registered after April 2017.

Older diesel cars (pre-2017) may have higher emissions-based VED rates. It's worth checking the exact rate for any used diesel you're considering on the DVLA website.

Clean Air Zones

Several UK cities operate Clean Air Zones (CAZs) that charge older, more polluting vehicles for entry. In most schemes, Euro 6 diesel cars (generally 2015 onwards) are exempt — but older diesels may face daily charges of £7–£12.50 to enter city centres in Birmingham, Bradford, Bristol, and Bath, among others.

If you regularly drive in or through these cities, check whether your vehicle would be charged before buying a diesel.

Resale Values

Diesel car residual values have softened noticeably since 2017, when the "dieselgate" scandal and government rhetoric around clean air shifted buyer sentiment. While a well-maintained diesel still commands a reasonable price, equivalent petrol models are increasingly holding their value better — particularly smaller cars. Hybrids have the strongest residual values in most segments right now.

Which Should You Choose?

At current UK prices, diesel is cheaper to run per mile — saving roughly <strong>1.5p per mile</strong> compared to a petrol equivalent. That said, the bigger decision factors are how you drive and where you drive, since the higher purchase price of a diesel also needs to be recouped: