The fiscal / taxable horsepower or just tax horsepower was an early system to calculate road taxation rates for automobiles in a number of key Western European countries such as the UK, Germany, France, Belgium and Italy. Some U.S. states like Illinois also charged license plate purchase and renewal fees for passenger automobiles, based on taxable horsepower. The tax horsepower rating was typically computed not from actual engine power but by mathematical formulae based on cylinder dimensions etc, and varying per country. In the early twentieth century, fiscal power was reasonably close to real power – but as the internal combustion engine developed, real power output outpaced nominal taxable power by a factor of up to ten or more.
Britain The so-called RAC horsepower rating was devised in 1910 by the RAC at the invitation of the British government. The formula is:
where: is the diameter (or bore) of the cylinder in inches, is the number of cylinders
The formula was calculated from total piston surface area (i.e., "bore" only). The factor of 2.5 accounts for characteristics that were widely seen in engines at the time, such as a mean effective pressure in the cylinder of and a maximum piston speed of .
The system introduced a somewhat progressive way of taxing higher-value cars more than low-cost ones but was also introduced to protect the domestic British motor industry from foreign imports, especially the Ford Model T. Henry Ford's mass production methods meant that the Model T was competitively priced with British-built cars despite being a much larger, more durable and more powerful car than other available similarly-priced models. In 1912 Ford opened a factory to build Model Ts in Manchester, to circumvent the import tariffs that, up to that point, had increased the effective price of foreign cars. Under the RAC's formula the Model T was a 22 'tax horsepower' car, making it more expensive to run than its British-built rivals on sale for the same price.
At first the RAC rating was usually representative of the car's actual (brake) horsepower, but as engine design and technology progressed in the 1920s and 1930s these two figures began to diverge, with engines making much more power than their RAC ratings suggested: by 1924 the engine of the Austin Seven (named for its 7 hp rating) produced , 50 percent more than its official rating.
It became common for the name of a model to include both its RAC tax horsepower and its actual power output, such as the Wolseley 14/60 and the Alvis 12/70 of 1938.
To minimise tax ratings British designers developed engines with very long stroke and low piston surface area. Another effect was the multiplicity of models: Sevens, Eights, Nines, Tens, Elevens, Twelves, Fourteens, Sixteens etc., each to fit with a taxation class.Motor-Car Taxation. Effects of Different Methods on Design. Models for Home Use and Export. The Times, Saturday, 16 Feb 1946; pg. 5; Issue 50378 Larger, more lightly stressed engines may have been equally economical to run and, in less variety, produced much more economically. The system discouraged manufacturers from switching to more fuel-efficient overhead valve engines as these generally required larger bores, while the established sidevalve layout could easily use very narrow bores. Despite OHV engines having significant benefits in economy, refinement and performance, the RAC system made these engines more expensive to own because it placed them in a higher tax class than sidevalve engines of identical power output. Despite this, by 1935 the Standard Flying Twelve, a typical mid-size saloon, produced from a engine, nearly four times as much power as the RAC system suggested.
While the RAC system had protected the home market from the import of large-engined low-priced mass-produced American cars, the need for roomy generously proportioned cars for export was now paramount and the British government abandoned the tax horsepower system with effect from 1 January 1947Change in Motor Tax on 1 January. The Times, Wednesday, 13 Feb 1946 replacing it at first with a tax on cubic capacity, which was in turn replaced by a flat tax applying from 1 January 1948.The New Car Tax. The Times, Wednesday, 18 Jun 1947 However British cars and cars in other countries applying the same approach to automobile taxation continued to feature long, relatively narrow cylinders even in the 1950s and 1960s, partly because limited investment meant that new car models often had new bodies but their engines were carried over from earlier generations.
The emphasis on long strokes, combined with the nature of British roads in the pre-motorway era, meant that British engines tended to deliver strong low- and mid-range torque for their size, but low maximum speeds. The long stroke also meant that piston speeds and the load on the big end bearings became potentially damaging at high power outputs. Many smaller British cars did not cope well with sustained cruising at or more, which led to reliability problems when the vehicles were exported to other markets, especially the United States. Cars such as the Austin A40, the Morris Minor and the Hillman Minx all achieved notable initial sales success in the US in the late 1940s, until the short service life of the engines when asked to routinely drive long distances at freeway speeds became clear. Other imports originating in countries with different tax rules and existent high speed road networks, in particular the Volkswagen Beetle, proved more reliable, and achieved greater sales success.
Australia In Australia, the various states had their own automobile taxation system. Several depended on the RAC formula, but the flaws of this system were well known as early as 1909. Another formula was the Dendy-Marshall formula, which included an engine's stroke. Several Australian states used Dendy-Marshall, although Western Australia reverted to RAC hp in 1957. The Union of South Africa also depended on Dendy-Marshall, at least in the 1920s. The Automobile Club of Australia's "A.C.A. formula" used the same calculations as did Dendy-Marshall formula. The Australian Bureau of Statistics used RAC hp in their Registration of New Motor Vehicles, Make of Vehicles, Australia statistics until this publication was discontinued in June 1976.
Several Australian states also added vehicle weight to the power rating, to get a power-weight unit which determined taxation.
The Dendy-Marshall / A.C.A. formula is as follows:
where: = nominal horsepower = the diameter (or bore) of the cylinder in inches = the length (or stroke) of the cylinder in inches = the number of cylinders
United States
During the early twentieth century, automobiles in the United States were specified with a figure identical to RAC horsepower and computed using the same formula; this was known either as "NACC horsepower" (named for the National Automobile Chamber of Commerce), "ALAM horsepower" (for NACC's predecessor, the Association of Licensed Automobile Manufacturers), or "SAE horsepower" (for the Society of Automotive Engineers). (This last term should not be confused with later horsepower ratings by the SAE.) This value is still used for taxation and license fee purposes in the State of Missouri for passenger vehicles, with electric vehicles assigned to the 12–23 Horsepower bracket by statute.
Continental Europe Although tax horsepower was computed on a similar basis in several other European countries during the two or three decades before World War II, continental cylinder dimensions were quoted in millimetres. As a result of rounding when converting the formula between the two measurement systems, a British tax horse-power unit ended up being worth 1.014 continental (i.e. French) tax horse-power units.
Belgium In Belgium, the tax power depended on the cubic capacity of the engine in cubic centimetres at the rate of 1 tax horse power for every .
France
The Cheval Fiscal, often abbreviated to CV from "chevaux-vapeur" (literally "steam horses") in tax law, is used for the issuing of French registration certificates known as "cartes grises" ("grey cards"). It is an administrative unit originally calculated partly from the power of the engine and used to calculate the amount of tax that may be due at the time of registration.
The Citroën 2CV (two tax horsepower) was the car that kept such a name for the longest time.
Its use in France dates from 1 January 1913. It was updated in 1956, with further revisions in 1978 and a new emission-based system introduced in 1998.
It was originally defined using the following formula:
where: is the number of cylinders, is the cylinder bore in centimetres, is the stroke in centimetres, is the maximum engine speed in revolutions per second, derived from manufacturers' performance claims, is a coefficient depending on the number of cylinders (single-cylinder engine : 0.00020; two-cylinder engine : 0.00017; four-cylinder engine: 0.00015; six-or-more-cylinder engine: 0.00013)
1956 formula In a circular issued on 28 December 1956, the cheval fiscal was defined as:
where: = puissance administrative, = constant of 0.00015, = number of cylinders, = bore in centimetres, = stroke in centimetres, = speed of rotation in revolutions per second
The result is multiplied by 0.7 for a four-stroke, diesel engine (also for wood gas-powered vehicles or those running on CNG). Since and are both constants while , , and combine to form the engine's displacement, a commission simplified the formula to:
where: = puissance administrative, = displacement in litres (1 L = ), = constant of 5.7294 for petrol engines and 4.0106 for diesel engines
1978 formula +1978 Engine Speed Parameter Formulas For calculating , k1, k2... kn represent the theoretical speed in km/h of the vehicle with the engine running at 1000 rpm in 1st, 2nd... nth forward gears. is a representation of the average speed across the forward gears, with various adjustments added for various transmission types. Forward speeds Manual transmissions Forward speeds Automatic transmissions 3 speeds(or fewer) 2-speed 4 speeds 3-speed 5 speeds k5 ≤ 1.25*k4: 4-speed k4 ≤ 1.4*k3: k5 > 1.25*k4: k4 > 1.4*k3: Manual w/Overdrive For transmissions with one or more overdrive speeds, the over-drive ratio is only applied to the fourth (top) gear. The calculationsare administered as for a five-speed transmission. CVT km and kM represent the velocity at 1000 rpmreached in the lowest and highest available ratios
A new system was announced on 23 December 1977 to come into force on 1 January 1978 calculated by the following formula:
where: = puissance administrative, = coefficient of 1 for petrol or 0.7 for diesel, = engine capacity in cubic centimetres, = parameter calculated from a weighted arithmetic average speed in kilometres per hour theoretically reached by the vehicle at an engine speed of 1000 revolutions per minute in each gearbox ratio (see dropdown table to the right for more details).
1998 formula From 1998 until January 2020, the fiscal tax depended on a standardised value of carbon dioxide emissions in grams per kilometre (g/km) and the maximum engine power in kilowatts (kW). If C is the amount of released and P the engine power, then:
is expressed in horsepower, rounded to the nearest integer. The official emission rate of included in the calculation is taken from the European certificate of conformity.
Germany Tax horse-power (Steuer-PS) was introduced in Germany on 3 June 1906 however in contrast to many regions, i.e. British and French tax horsepower formulae above, it was calculated based on the overall engine displacement from its implementation.