Petrol vs Diesel costs: compare the whole journey

A litre of diesel can cost more than a litre of petrol while a diesel car still spends less on fuel per mile, because the vehicles may use different amounts. The reverse can also be true when the diesel efficiency advantage is small or the pump-price gap is large.

Compare specific vehicles, not an imaginary average petrol car with an imaginary average diesel car. Engine size, vehicle weight, transmission, route and driving style can matter more than the badge on the fuel cap.

Start with miles per gallon and the current price per litre. Convert the imperial gallon to 4.546 litres. The calculation is: price per litre multiplied by 4.546, divided by miles per gallon. Keep the units consistent and compare the result in pence per mile.

For example, a vehicle returning 45 mpg at 140p/l costs about 14.1p per mile in fuel. A vehicle returning 55 mpg at 147p/l costs about 12.2p per mile. The more expensive litre produces the cheaper mile in that example, but different real economy can reverse it.

A 2p-per-mile difference is about £100 over 5,000 miles and £300 over 15,000 miles. That gives you a ceiling for how much extra purchase, finance, maintenance or insurance cost the fuel saving could reasonably offset.

Use a realistic annual mileage and separate motorway, rural and town driving if your pattern is mixed. Short-trip use can be a poor fit for some diesel emissions systems, while regular long journeys may suit them better. Vehicle-specific advice and maintenance history matter.

Purchase price, depreciation, tax, insurance, servicing, repairs and clean-air-zone eligibility can outweigh the fuel calculation. Those costs belong to the particular vehicle and where it will be driven, not to petrol or diesel as universal categories.

Use current petrol prices and diesel prices as inputs, then make the ownership decision with vehicle-specific figures. PetrolDiesel compares fuel prices, not the mechanical condition or total value of a car.