Diesel or petrol: which is worth it?
Diesel usually costs more to buy but uses less per kilometre. Enter your annual mileage, consumptions and prices to see which is worth it and how many km it takes to recoup the diesel's extra cost.
How much more the diesel costs to buy
Frequently asked questions
When is buying a diesel worth it?+
When you drive a lot per year. Diesel saves on fuel but costs more to buy; from ~15,000–20,000 km/year it usually starts to pay off.
What about diesel restrictions in cities?+
Pre-2015 diesels (without a C sticker) have limited access to Low-Emission Zones. If you drive a lot in town, check the sticker before deciding.
How are the diesel payback years calculated?+
You divide the diesel's purchase premium by the yearly fuel saving (km per year × cost-per-km difference). If you recover that premium in about 5 years or less, diesel usually pays off; beyond that, petrol is the more sensible choice.
What if I don't drive many kilometres a year?+
With low mileage the annual saving isn't enough to recoup the premium. With the form's default values (a saving of about €0.03/km), at 5,000 km/year you save ~€149 a year and would need over 13 years to recover a €2,000 premium: petrol wins clearly.
What matters more, the price per litre or the consumption?+
Both at once: what decides it is the cost per kilometre, which comes from multiplying consumption (L/100 km) by price (€/L) and dividing by 100. A diesel can pay off even with the litre at the same price as petrol, if its lower consumption keeps the cost per km below.
Where do I check real petrol and diesel prices?+
On benzly's fuel station map you have up-to-date prices at stations near you, and in the observatory you can compare the average by region. Use the prices you actually pay in the form: the result will be far truer than with generic averages.
What does the break-even point in kilometres mean?+
It's the total kilometres — not per year — you need to drive for the fuel saving to equal the diesel's purchase premium. It's worked out by dividing the premium by the saving per kilometre. Beyond that point, every extra kilometre is money in the diesel's favour.
The logic of the maths: premium versus saving per kilometre
In Spain a litre of diesel is usually a bit cheaper than petrol — the form's defaults use €1.55 versus €1.65 — and diesel engines burn fewer litres per 100 km. The trade-off sits at the dealership: the diesel version of the same model costs more, and that difference is the premium you enter in the form.
So the useful question isn't which is better, but how many kilometres you need for the saving to pay off the premium. That's exactly what this calculator answers, in four steps:
- Cost per km of each engine: consumption (L/100 km) × price per litre ÷ 100.
- Annual saving: the cost-per-km difference multiplied by your kilometres per year.
- Break-even point: the diesel's premium divided by the saving per km — the total kilometres it takes to recover it.
- Verdict: if you recoup it in about 5 years or less, the diesel pays off; if it takes longer — or if the diesel doesn't even save per km — petrol wins.
Worked example with the form's values
With the defaults — 15,000 km a year, consumptions of 6.5 and 5.0 L/100 km, petrol at €1.65/L, diesel at €1.55/L and a €2,000 purchase premium — the maths goes like this:
Every figure comes from the formulas in the previous section, so you can reproduce them with any calculator; changing a single field in the form recalculates the whole chain instantly:
- Cost per km: €0.107 for petrol versus €0.078 for diesel — about €0.03 saved per km.
- Annual saving: €0.03/km × 15,000 km ≈ €446 a year on fuel.
- Break-even point: €2,000 ÷ €0.03/km ≈ 67,000 km in total.
- Payback: €2,000 ÷ €446/year ≈ 4.5 years → verdict: diesel.
Mileage: the variable in charge
The same car flips its verdict just by moving the mileage: with the example's figures, at 25,000 km/year the premium is recouped in about 2.7 years (clearly diesel); at 8,000 km/year it stretches to 8.4 years and stops paying off. Hence the classic rule of ~15,000-20,000 km/year as the diesel's rough threshold: below it, the per-km saving rarely repays the premium within a reasonable time.
Remember the break-even point is total kilometres, not annual: 67,000 km at 15,000 km/year is about four and a half years of ownership. If you tend to change cars before recouping, the premium is never recovered — and the calculator's verdict will tell you straight: petrol.
There's one case where kilometres don't even come into it: if with your consumptions and prices the diesel doesn't save per kilometre, there is no break-even point. The calculator then shows the cost-per-km comparison and the verdict goes straight to petrol.
Use real prices and check the sticker too
The price gap between petrol and diesel isn't fixed: it moves with the market, between regions and from station to station. Since the maths multiplies consumption by price, a few cents per litre shift the break-even point by thousands of kilometres. Swap the defaults for the prices you actually pay: they're kept up to date on the fuel station map and, for the picture by territory, on prices by region.
Fuel isn't the only variable in the decision either. Diesels registered before September 2015 don't reach the C sticker and have limited access to Low-Emission Zones: if your daily driving is urban, check which environmental sticker the car would get before deciding.
And if you're weighing more than two energies, complete the picture with the EV charging cost and the fuel cost calculator.