Petrol vs diesel in 2026: which is cheaper to run right now?

The old rule was simple: diesel costs more per litre but you go further on it, so you win on high-mileage journeys. In 2026 that rule needs updating.

The pump-price gap

Diesel currently trades a few pence per litre above unleaded across most of the UK. In some weeks it's much wider. That instantly eats into the MPG advantage.

The MPG advantage

A modern diesel still returns roughly 15–25% better real-world MPG than a comparable petrol on longer runs. On short town-only trips the gap is much smaller — diesels don't warm up efficiently in a 3-mile school run.

The costs people forget

• ULEZ and Clean Air Zones — pre-2015 diesels get hit; petrols from 2006 onward are usually exempt.
• Servicing — DPFs, EGRs and AdBlue on modern diesels are real, real, real expensive when they fail.
• Resale — buyer appetite for used diesels has cooled, especially anything under 2.0 litres.

So which wins?

Rough rule of thumb for 2026: if you do under 10,000 miles a year, mostly urban, petrol is almost always cheaper overall. Over 15,000 miles a year on motorways, diesel still wins. In between, it depends on the exact car — do the sums with your actual annual mileage, not the sticker MPG.