kW, hp and Nm: how to read a truck's power figures

A tractor unit’s specification line reads something like “390 kW / 530 hp, 2,600 Nm”. Three numbers, two of which are the same number, and none of which tells you directly whether the truck will hold 85 km/h up the Brenner at 40 tonnes. They are all useful, but only once you know what each one is measuring and where it was measured.

Two of the three numbers are the same number

Kilowatts and horsepower are the same quantity in different units. In the EU the kilowatt is the legal one: Council Directive 80/181/EEC makes the watt the legal SI unit of power, and anything else printed beside it is a “supplementary indication” which must appear in characters no larger than the legal unit. The type-approval paperwork is in kilowatts. The horsepower figure is a courtesy to the market.

There are also two horsepowers. Metric horsepower — PS in Germany, ch in France, pk in the Netherlands — is 735.5 W. Imperial brake horsepower is 745.7 W. So 390 kW is 530 PS but only 523 bhp, a gap of seven that is entirely an artefact of which country wrote the brochure. Continental truck makers use the metric one, and it is why model names look the way they do: a Scania badged 560 is running 412 kW, MAN’s 640 is 471 kW, a Volvo FH 540 is 397 kW, and a Mercedes Actros L 1845 is 450 PS — 330 kW — with the 18 in front denoting the weight class rather than anything about the engine.

If you are comparing trucks across markets, convert everything to kilowatts first. It is the only figure that means the same thing in every brochure.

Where the figure comes from, and what “net” excludes

Engine power for type approval is net power, and the definition is precise. UN Regulation No 85 defines it as the power obtained on a test bench at the end of the crankshaft, with the auxiliaries listed in the regulation’s annexes fitted, and determined under reference atmospheric conditions. For heavy-duty engines in the EU the measurement runs under Annex XIV of Regulation (EU) No 582/2011, which adopts the UNECE method.

Two consequences follow. First, the figure is genuinely comparable between manufacturers — it is a homologated measurement, not a marketing claim, and the engine-mounted auxiliaries are already subtracted. Second, it is measured at the crankshaft, so the driveline has not taken its cut yet. Nothing on the specification sheet tells you what arrives at the tyre.

Torque is the number; power is arithmetic done to it

Power is not an independent property. It is torque multiplied by rotational speed:

P (kW) = T (Nm) × n (rpm) / 9,549

Which means the two headline numbers are a single curve sampled at two different points. Scania’s Super 13-litre makes full torque of 2,800 Nm from 900 rpm and its rated 412 kW at 1,800 rpm. Run the arithmetic: at 900 rpm those 2,800 Nm are 264 kW. By rated speed the engine is producing 412 kW, but torque has fallen to about 2,186 Nm. Torque dropped by a fifth while power rose by more than half, because the revs doubled.

That is the whole relationship. Torque is what resists the gradient at a given moment. Power is torque combined with how fast you are turning, and therefore what sets how quickly you can do work — how fast you climb, not whether you climb.

For a long-haul tractor the useful question is not the peak of either curve but how wide the torque plateau is and how low it starts. Mercedes quotes maximum torque at 1,100 rpm across the Actros L range, with Top Torque adding a further 200 Nm in twelfth gear. That last detail is worth more than its size suggests: it is what lets the truck hold top gear on a rise instead of dropping a gear, losing momentum and burning fuel recovering it.

Rated speed, and why a low one is a selling point

Rated speed is the engine speed at which peak power is reached, and comparing it across makers is more informative than comparing the power itself. PACCAR’s MX-13 in the DAF XG+ is rated at 1,600 rpm; Scania’s 13-litre and MAN’s D38 are rated at 1,800. Lower rated speed with comparable power implies more torque doing the work, and it usually comes with taller final drive gearing.

This is downspeeding, and it is the dominant fuel strategy in European long-haul: cruise at 1,000 to 1,200 rpm, where the engine is most efficient, and rely on a fat torque plateau plus an automated gearbox with enough ratios to stay there. It only works if torque at those revs is genuinely high. A truck with an impressive peak power figure and a narrow, high-revving torque band will shift constantly on undulating ground and consume more than a nominally weaker one.

The legal floor: 5 kW per tonne

There is a statutory minimum, and it is lower than most people assume. Commission Regulation (EU) No 1230/2012, Annex I Part C, requires N2 and N3 vehicles to provide at least 5 kW per tonne of the technically permissible maximum laden mass of the combination. At 40 tonnes that is 200 kW; at 44 tonnes, 220 kW. Road tractors built for indivisible loads get a relaxed floor of 2 kW per tonne. The same annex requires a vehicle designed to tow a trailer, laden to its maximum combination mass, to start five times within five minutes on a gradient of at least 12%.

Every tractor in a mainstream European catalogue clears 200 kW comfortably — the lightest engines on offer start at around 240 kW. So the question is never whether a truck is legal, but what you are buying with the 100 to 250 kW you are adding above the floor.

Which number matters for which job

For 40 tonnes on made roads with normal topography, something in the region of 330 to 370 kW is not a compromise; it is the right answer. More power costs purchase price, weight and usually fuel, and it buys schedule margin you may not need.

Weight-critical work argues the other way entirely. The nine- and eleven-litre options — MAN’s D15 at 243 to 294 kW, PACCAR’s MX-11, the Mercedes OM 470 — exist because a lighter engine block is payload, and on a tanker or a bulk tipper running to a weight limit that is revenue.

Sustained gradients, 44 tonnes and up, or Scandinavian combinations well beyond that are where the big blocks earn their keep. The 16-litre V8 in the Scania S-series runs to 566 kW, MAN’s D38 to 471 kW, and Mercedes’ OM 473 to 460 kW with 3,000 Nm. Buy them for the torque and the thermal headroom on long climbs, not for the top speed.

What the power figure cannot tell you

It cannot tell you fuel consumption. Since 2019 that has been certified separately for the whole vehicle under Regulation (EU) 2017/2400 using the VECTO simulation, which accounts for aerodynamics, tyres, axle ratio and transmission alongside the engine. Two tractors with identical kilowatt ratings can post meaningfully different declared consumption.

It also cannot tell you gradeability at your gross weight, because that is a function of gearing as much as of the engine. If a hill is the reason you are specifying the truck, ask for the startability and gradeability figures for the exact driveline, not the engine rating.