Petrol vs diesel: what every driver should know

Petrol and diesel aren't interchangeable, and the differences affect cost, performance and where you'll find each. Here's the practical version.

The core difference

Petrol (gasoline) ignites with a spark; diesel ignites under compression. That mechanical difference drives everything else: diesel engines are generally more efficient and produce more low-end torque, while petrol engines are lighter, rev higher and are cheaper to build and maintain.

Which suits your driving

• Lots of long-distance and motorway miles: diesel's efficiency usually pays off, and its torque is well suited to steady high-speed cruising and towing.
• Mostly short urban trips: petrol is often the better fit — diesel particulate filters dislike short journeys that never get hot enough to clean themselves.
• Occasional, mixed use: modern petrol (and hybrids) usually make more sense on total cost.

Availability and grades

Both are widely available, but the grade names vary by country. Petrol is sold by octane rating (91/95/98 RON, or 87/89/93 AON in North America); diesel is often split into regular and premium, plus seasonal summer/winter blends in cold climates. When you search the map, filter by the specific fuel you need so you only see stations that sell it.

Cost picture

Diesel sometimes costs more per litre than petrol and sometimes less, depending on the country and season — the pump price alone doesn't tell you which is cheaper to run. The real comparison is cost per mile, which factors in the engine's efficiency and the vehicle's price. For a specific car, that's the number worth working out.

Before you fill up

Check your fuel filler cap or handbook for the correct fuel and minimum grade. Using a higher grade than your car needs rarely helps; using a lower grade than specified can. When in doubt, the manufacturer's minimum is the safe, economical choice.