The tax horsepower or taxable horsepower was an early system by which taxation rates for
automobiles were reckoned in some European countries, like Britain, Belgium, Germany, France, Italyand certain US states, (such as Illinois), where license platepurchase and renewal fees for passenger automobiles were based on taxable horsepower. The tax horsepowerrating was computed not from actual engine power but by a simple mathematical formula based on cylinder dimensions. At beginning of the twentieth century tax power was reasonably close to real power; as the internal combustion engine developed, real power became larger than nominal taxable power by a factor of ten or more.
The British system was based on the RAC horsepower rating which was calculated from total piston area only. To minimise tax rating British designers developed engines of a given swept volume (capacity) with very long stroke and low piston area, a custom which continued long after taxation ceased to be based on piston area.
The tax horsepower rating was often used as the car model name. The
Citroën 2CV(French "deux chevaux vapeur", two steam horses) was the car that kept such a name for the longest time.
Taxation can modify incentives and tax horsepower is no exception. Large capacity (displacement) engines are penalized, so engineers working where engine capacity is taxed are encouraged to minimize capacity. This hardly happened in the USA, where license plate fees, even adjusted for horsepower ratings, were comparatively much lower than European auto taxes.
French-made vehicles after
World War IIin particular have had very small engines relative to vehicle size. The very small Citroën 2CVfor example has a 425 cc two cylinder engine that weighs only convert|100|lb|kg, and at the opposite extreme, the large Citroën SMhas a still modest 2,700 cc six cylinder engine that weighs only convert|300|lb|kg.
Cars with engines of small displacement often have better fuel economy than comparable cars with larger engines for several reasons. First, the lower engine weight improves economy. Second, small-displacement engines tend to be designed with fewer cylinders and correspondingly fewer parts, which helps to reduce internal friction. Third, to produce the same amount of power, a smaller engine runs at higher throttle openings and manifold pressures than a larger engine running at the same RPM. This reduces pumping losses, the energy the engine must expend to pull in air past the throttle into the cylinders, akin to sucking a
milkshakethrough a thin straw (part throttle) or a thick straw (open throttle). Fourth, smaller engines lead to smaller and more aerodynamically efficient body designs, further improving fuel economy. While it is possible for larger engines to achieve good fuel economy, it is easier and cheaper with a smaller engine.
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