Petrol or diesel: which is cheaper for your annual mileage?

The short answer

• A diesel uses roughly 17–26% less fuel than a comparable petrol car.
• At today’s prices, a typical family hatchback saves about €1.23 every 100 km with diesel: if it costs €2,500 more, it takes around 203,000 km to win that back.
• At 10,000 km (6,200 miles) a year, winning back that €2,500 takes about 20 years. If your mileage is low or most of your trips are short, petrol is usually the cheaper choice.
• Fuel isn’t the biggest factor: the difference in purchase price and in what each car is worth when you sell it matter more. Always compare specific models.

How much less fuel a diesel uses

According to fueleconomy.gov, a diesel goes 20–35% further on the same amount of fuel than a comparable petrol car, which means it uses roughly 17–26% fewer litres. Part of that comes from the fuel itself, which holds 10–15% more energy per litre, and part from the engine, which makes better use of it.

Real-world data from the European Commission points the same way: new cars registered in 2021 averaged 7.89 L/100 km on petrol and 6.88 L/100 km on diesel, even though the diesels were heavier (1,747 kg on average against 1,404 kg). Those averages mix different models, so they can’t be used to compare one particular car, but they confirm the advantage.

Either way, real-world consumption is higher than the official figure: by 20% for petrol and 17% for diesel, according to the same data. We explain why in real-world vs official fuel economy.

Diesel prices and tax

For years, diesel had a second advantage: the fuel was much cheaper. On tax alone, it should be. Spanish fuel duty is €0.473 per litre of petrol and €0.379 per litre of diesel; with 21% VAT on top, petrol pays about 11 cents more per litre.

Yet on 7 November 2026 unleaded 95 averaged €1.678/L and diesel €1.872/L at Spanish filling stations: diesel is about 19 cents a litre dearer. Before tax, diesel is dearer, and that wipes out its tax advantage. So today a diesel’s savings come almost entirely from using fewer litres.

How many kilometres it takes to win back the extra cost

Take a typical family hatchback rated at 6.0 L/100 km WLTP as a petrol and 4.8 L/100 km as a diesel. With the calculator’s correction, that’s about 7.2 and 5.6 L/100 km in real life. At today’s average prices, and including AdBlue, the diesel saves about €1.23 every 100 km.

If diesel went back to the price its lower tax suggests, about 11 cents below petrol, the saving would rise to about €2.96 every 100 km and the payback times would shrink accordingly.

Consumer organisations reach similar conclusions. In 2023, when diesel was 15 cents a litre cheaper than petrol, Spain’s OCU worked out that a diesel costing €2,500 more pays for itself after about 75,000 km. Germany’s ADAC, which compares around 270 petrol and diesel model pairs with all costs over five years, concludes there is no general rule: it depends on the model, and depreciation is the biggest cost.

What you don’t see at the pump

• AdBlue: modern diesels need it to cut nitrogen oxides. The ADAC puts consumption at 3–5% of the diesel you use, and it costs €1–2 a litre at filling stations. It’s already included in the table.
• Diesel particulate filter: it cleans itself by burning off soot, which needs exhaust gas above 550 °C. Lots of short trips never get it that hot, and the filter can clog or be damaged; the ADAC puts a replacement at €1,000–3,000.
• Registration tax in Spain: it depends on official CO₂ emissions. Up to 120 g/km it is 0%; above 120 and below 160 g/km it is 4.75% (on the mainland and the Balearics, unless the region has set its own rate). If one version crosses that line, on a car costing €20,000 before VAT the difference is €950.
• DGT environmental label: any new car, petrol or diesel, meets Euro 6 and gets the C label. With used cars it does matter: petrol cars get the C label from Euro 4, but diesels only from Euro 6; a Euro 4 or 5 diesel gets the B label.

Tips

• Work out your real annual mileage from your service or MOT records, and your fuel economy with the brim-to-brim method.
• Compare specific versions of the same model: final price, WLTP consumption and CO₂ emissions (which set the registration tax in Spain).
• Multiply the difference in consumption by the kilometres you’ll drive for as long as you keep the car, not just one year.
• If most of your driving is short trips around town, think twice about a diesel even if your mileage is high: the particulate filter needs longer runs to clean itself.
• For a specific journey, try both fuels in the calculator with today’s fuel price.

Frequently asked questions