Fiscal Horsepower Is Not Horsepower

Nobody measured it

An engine has a power output, and it is measured on a dynamometer to a published test procedure. Fiscal horsepower is not that number. It is an administrative quantity, defined by a legal text, whose only purpose is to sort vehicles into tax bands. It is computed from things that were measured — kilowatts, cubic centimetres, grams of CO2 per kilometre — but the computation itself is a policy choice, and it can be, and has been, rewritten by a finance act.

That matters more than it sounds, because it means two things people routinely assume are false. Fiscal horsepower is not convertible to real horsepower by any fixed factor: two engines of identical output can carry different fiscal figures if they differ in displacement, cylinder count or emissions, depending on which country's formula you are in. And a car's fiscal figure is not a property of the car — it is a property of the car under a particular country's rules at a particular date. Change the country or the date and the number changes with it.

France: the CO2 formula is the old one

France is the market that gave the concept its name — chevaux fiscaux, the CV that appears in field P.6 of the registration document — and it is also the market where the formula has changed most often. The version in force for cars first registered from 1 January 2021 uses net maximum engine power and nothing else: PA = 1.80 × (PM ÷ 100)² + 3.87 × (PM ÷ 100) + 1.34, with PM in kilowatts, rounded to the nearest whole number. It is written into the Code des impositions sur les biens et services, article L421-16.

The formula everyone still quotes — CO2 ÷ 45 + (kW ÷ 40)^1.6 — is the previous one. It governs cars first registered in France between 1 July 1998 and the end of 2020, and it lives on in the same code among the articles that preserve the old rules. Older cars again fall under a displacement-based rule, computed from engine capacity with a transmission factor and a reduction for diesel. Three formulas, three eras, all still in force on the roads at the same time.

On our example car the two modern formulas happen to agree. The power-only version gives 1.80 × 1.21 + 3.87 × 1.10 + 1.34 = 7.775 and the CO2 version gives 132 ÷ 45 + (110 ÷ 40)^1.6 = 2.933 + 5.046 = 7.979; both round to 8 CV. They part company as power rises. At 250 kW and 250 g CO2/km the modern formula gives 22.265, so 22 CV, while the old one gives 24.323, so 24. At 375 kW and 300 g/km it is 41 against 43. The change was not cosmetic; it moved money between kinds of car. Electric vehicles have their own power-only formula, in article L421-20, which was itself amended with effect in 2026 — another reason to read the current text rather than a remembered one.

Spain: a formula written in bore and stroke

Spain has fiscal horsepower too, under the name caballos de vapor fiscales, but it has nothing in common with the French version. Annex V of Real Decreto 2822/1998 defines it, for a four-stroke engine, as CVF = 0.08 × (0.785 × D² × R)^0.6 × N, where D is the cylinder bore in centimetres, R the piston stroke in centimetres and N the number of cylinders. Two-stroke engines use 0.11 instead of 0.08, and rotary and electric motors use effective power divided by 5.152.

Look at what 0.785 × D² × R actually is: it is πD²R ÷ 4, the swept volume of one cylinder. So the whole formula reduces to 0.08 × (displacement per cylinder)^0.6 × N, and you never need bore and stroke at all — total capacity and cylinder count are enough. Our 1,498 cm³ four has 374.5 cm³ per cylinder; 374.5^0.6 = 35.000, times 0.08 times 4 gives 11.20 fiscal horsepower.

The exponent has a consequence worth spelling out. Because 0.6 is applied per cylinder and the result is then multiplied by the cylinder count, splitting the same capacity into more cylinders raises the fiscal figure. The same 1,498 cm³ arranged as a six gives 13.17 CVF instead of 11.20 — a whole band higher in the annual municipal tax. That tax, set out in article 95 of the local finance law, charges a base amount by band: under 8 CVF, 8 to 11.99, 12 to 15.99, 16 to 19.99, and 20 and above. Each municipality may multiply the base by a coefficient, which the law caps at 2, so two identical cars a few kilometres apart can pay twice the difference.

Spain's registration tax works on something else entirely. The special tax on certain means of transport is charged as a percentage of the taxable value, banded by official CO2: nothing at 120 g/km or below, 4.75% above 120 and below 160, 9.75% from 160 to below 200, and 14.75% at 200 and above — those being the default peninsular rates where the autonomous community has not set its own. Our example car at 132 g/km falls in the 4.75% band. So a Spanish buyer meets two completely different tax bases on the same day: a bore-and-stroke formula for the annual tax and an emissions band for the purchase.

Germany: displacement plus a CO2 staircase

Germany has no fiscal horsepower and never adopted the concept. The motor vehicle tax law charges a passenger car on two independent components. The first is displacement: 2 euros for each started 100 cm³ on a spark-ignition engine and 9.50 euros on a compression-ignition one, which is why the diesel-versus-petrol arithmetic in Germany is dominated by that single factor of nearly five. The second is CO2, applied only above an allowance of 95 g/km for cars first registered from 1 January 2021, in six marginal bands: 2.00 euros per gram from 95 to 115, then 2.20, 2.50, 2.90, 3.40 and 4.00 euros per gram as the emissions climb through 135, 155, 175 and 195 g/km.

Run our car through it. 1,498 cm³ is fifteen started hundreds, so 15 × 2 = 30.00 euros. Its 132 g/km sits in the first two bands: 20 grams at 2.00 plus 17 grams at 2.20 gives 40.00 + 37.40 = 77.40 euros. The annual bill is 107.40 euros. Change nothing but the engine — a 1,968 cm³ diesel at 140 g/km — and the displacement half becomes 20 × 9.50 = 190.00 euros and the CO2 half 96.50, for 286.50 euros a year. Almost all of that difference is the diesel rate on displacement, not the emissions.

Italy and Portugal: no fiscal horsepower at all

Italy taxes kilowatts. The annual bollo is charged per kilowatt of net maximum power, at a rate that depends on the emission class and is set and varied by each region; the widely used reference tariff for Euro 4 and later is 2.58 per kilowatt up to 100 kW and 3.87 per kilowatt beyond. Our 110 kW car gives 100 × 2.58 + 10 × 3.87 = 296.70 euros a year at that reference. Above 185 kW a national surcharge applies on top: the addizionale erariale, universally called the superbollo, is 20 euros per kilowatt of power above 185 kW, reduced to 60% of that after five years from the date of construction, 30% after ten, 15% after fifteen, and dropped entirely after twenty. A 200 kW car therefore pays 645.00 for the bollo plus 300.00 for the superbollo — 945.00 euros a year — until it starts ageing out of the surcharge.

Portugal splits the question in two. The vehicle tax paid on first registration, the ISV, is the sum of a cylinder-capacity component and an environmental component computed on CO2, with a surcharge on diesel engines, a relief tied to particulate emissions and a percentage reduction by age for used vehicles brought in from elsewhere. The annual circulation tax is separately banded on capacity and CO2. Both rate tables are re-set by each year's state budget, so the honest thing to say here is the structure rather than the numbers: look up the current tables at the moment you register, because a table quoted from memory is a table from a different year.

Why the same car costs different amounts to put on the road

Because each market taxes a different quantity, the ranking of cars changes as you cross a border. A big-displacement, low-power engine is punished in Germany and Portugal and treated gently in Italy. A small-displacement, high-power turbo is the reverse. A six-cylinder is penalised in Spain relative to a four of the same capacity, purely because of that 0.6 exponent, and is invisible to the formula in France and Italy. None of this is a comparison of the cars; it is a comparison of the formulas.

The French registration cost shows how far a single administrative number can travel. The regional tax is the fiscal horsepower multiplied by a rate per CV that each région sets and revises, on top of a fixed charge of 11 euros and a delivery fee of 2.76 euros. At 8 CV, a rate of 43 euros per CV gives 344 euros of regional tax and a rate of 60 gives 480 — the same car, the same paperwork, a difference decided by which side of a regional boundary the owner lives on. Since those rates move, and have moved sharply in recent years, the rate to use is always the one published for your région at the moment you register.

How to check your own number

Everything above starts from three figures that are printed on the vehicle's registration document: net maximum power in kilowatts, engine capacity in cubic centimetres, and combined CO2 in grams per kilometre. In the harmonised European format they are fields P.2, P.1 and V.7, and the administrative power, where the country uses one, is field P.6. If the P.6 figure disagrees with the formula for its registration date, the usual explanation is that the vehicle falls under an earlier rule rather than that the arithmetic is wrong.

Then take one precaution that applies everywhere. Every rate in this article is a rate from a text that a finance act can and does rewrite: a per-CV regional tariff, a euro-per-gram band, a euro-per-kilowatt tariff, a municipal coefficient. The formulas move more slowly than the rates but they do move, as France has demonstrated twice. Use a calculation to understand what your bill is made of, then check the current rate from the tax authority or the registration service before you rely on the total.

Frequently asked questions

How many kilowatts is one fiscal horsepower?

There is no conversion factor, because the relationship is not linear and is not the same in two countries. Under the current French formula, 55 kW gives 4 CV, 110 kW gives 8 and 250 kW gives 22 — so the kilowatts per CV are 13.8, 13.8 and 11.4 respectively. Any table claiming a fixed ratio is either quoting one country's formula at one power level or is simply wrong.

Does an electric car have fiscal horsepower?

Where the concept exists, yes, but computed by a separate rule. France gives electric vehicles their own power-only formula in article L421-20 of the tax code, distinct from the one used for combustion cars and amended with effect in 2026. Spain sends electric and rotary motors down a different branch of its annex, dividing effective power by 5.152. Since these provisions are being rewritten as electric registrations grow, read the article rather than a summary — including this one.

Why does my old car have a different fiscal power from a new one with the same engine?

Because the formula that applies is the one in force for the vehicle's date of first registration, not the one in force today. France alone has three generations of rule running side by side: a displacement-based one for cars first registered before July 1998, a CO2-and-power one for those registered from then until the end of 2020, and a power-only one since. Two cars with identical engines and different registration dates can legitimately carry different figures on their papers.

Is fiscal power the same thing as an insurance rating?

No, though insurers in some markets do use it as one input among many. An insurance rating is a commercial risk classification built from claims data — the model, the driver, the postcode, the repair cost. Fiscal power is a tax variable defined by statute. They can move in the same direction on the same car, which makes them easy to confuse, but a change in the tax formula does not change the risk of a claim and an insurer's tables have no authority over what you owe the tax office.

Which countries still use fiscal horsepower at all?

Of the five European markets covered here, France and Spain compute an administrative power and use it in a tax; Germany, Italy and Portugal do not have one at all, and tax displacement, kilowatts and CO2 directly. Because that list is a matter of national law and national law changes, the safe habit is to check the current basis for the country you are registering in rather than to assume the concept travels. It very often does not.