Your car costs you 30 cents a kilometre and you pay all of it

Thirty cents. That is what every kilometre of an ordinary private car costs, and the sum takes ten minutes with four pieces of paper you already have at home: the insurance receipt, the road tax one, the last garage invoice and the roadworthiness test.

And they are two numbers, not one: what it costs to OWN the car and what it costs to TAKE it. The first is those thirty cents; the second is between twelve and fifteen. Confusing them is what makes somebody drive for half an hour to save eight euros.

Take a petrol hatchback bought in 2019 for 14,500 euros and now advertised second-hand at 6,300. That is 8,200 euros lost in seven years: 1,171 a year, the biggest item of all, and the only one you never see leaving your account. Add the insurance, 390. The local tax, 92. The roadworthiness test, 21 a year: it costs about 42 euros and at this age the car takes it every two years. Servicing and tyres, 365. Total, 2,039 euros a year that you pay whether the car moves or not. If you drive 10,000 kilometres, that is 20.4 cents a kilometre before you start the engine.

Fuel, at 6.2 litres per hundred and 1.52 a litre, adds 9.4 cents.

Twenty-nine point eight. Thirty cents, rounded. And you count nine.

There is an official figure nearby. Since 17 July 2023, when a company sends an employee to another city in their own car, the Spanish tax office exempts 0.26 euros per kilometre from income tax; it used to be 0.19. Careful: the tax office does not pay it, the company does, and the order that sets the figure does not explain where it comes from. But it works as a check: if your sum comes out at 0.12, you have forgotten depreciation.

Back to the two numbers.

The thirty cents are the AVERAGE cost, and they are for deciding whether to own a car, not whether to take it. Insurance, tax and almost all the depreciation get paid even if you never leave the garage today: that money is already spent. What one extra trip costs is the MARGINAL cost: fuel plus the wear that trip causes — tyres, oil, brakes, and the share of depreciation that goes by kilometres rather than by years — somewhere between 12 and 15 cents a kilometre in a car like the one above.

Mixing the two is expensive in both directions. The average cost answers whether the household's second car, doing 4,000 kilometres a year and working out at more than 50 cents a kilometre in fixed costs alone, is worth keeping instead of renting twelve weekends. The marginal cost answers whether driving down to the industrial estate is worth it.

Do both sums once a year, the day the insurance bill arrives, and keep the two numbers on your phone. Then use them: the 24-kilometre round trip you make to save eight euros on the shopping costs you about 3.60 in fuel and wear, so you save 4.40, not eight. And if that car does 4,000 kilometres a year, the question on insurance day is not that one: it is whether those 2,039 euros buy you anything twelve weekend rentals would not.