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The tax horsepower or taxable horsepower was an early system by which taxation rates for automobiles were reckoned in some European countries, such as Britain, Belgium, Germany, France, and Italy; some US states like Illinois charged license plate purchase and renewal fees for passenger automobiles based on taxable horsepower. The tax horsepower rating was computed not from actual engine power but by a simple mathematical formula based on cylinder dimensions. At the 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. ==Britain== The so-called RAC horse-power formula was concocted in 1910 by the RAC at the invitation of the British government. The British RAC horsepower rating was calculated from total piston surface area (i.e. "bore" only). To minimise tax ratings British designers developed engines of a given swept volume (capacity) 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, Feb 16, 1946; pg. 5; Issue 50378〕 Larger more lightly stressed engines may have been equally economical to run yet, in less variety, produced much more economically.〔 The division by 2.5 in the RAC formula was supposed to account for the relatively low efficiency of early automotive engines and when first implemented in 1910 the RAC horsepower number was usually representative of the car's actual (brake) horsepower. However as engine design and technology progressed in the 1920s and 1930s these two figures began to drift apart, with an engine making much more power than its RAC rating (and the car's model name) suggested - even by 1924 the Austin Seven's 747cc engine produced 10.5 brake horsepower - 50 per cent more than its official rating. By the 1930s it was common, especially for more powerful cars, for 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 - note how the Alvis's engine makes more brake horsepower than the Wolseley but makes less power under the RAC system and thus would be taxed less. By 1948 the Standard Flying Twelve, a typical mid-size saloon, produced 44bhp from a 1.6-litre engine or nearly four times as much horsepower as the RAC system suggested, even though manufactureres were still restricted to making slow-revving engines with long bores and narrow strokes to keep abreast of the tax structure. The system also discouraged manufacturers from switching to more fuel-efficient overhead valve engines as these generally required larger bores, while the established sidevalve layout could easily utilise very narrow bores. Despite OHV engines having significant benefits in economy, refinement and performance the RAC system often made these engines more expensive to run due to placing an OHV engine in a higher tax class to a sidevalve engine of identical power output. British cars and cars in other countries applying the same approach to automobile taxation continued to feature these long thin cylinders in their engine blocks even in the 1950s and 1960s, after auto-taxation had ceased to be based on piston diameters, partly because limited funds meant that investment in new models often involved new bodies while under the hood/bonnet engines lurked from earlier decades with only minor upgrades such as (typically) higher compression ratios as higher octane fuels slowly returned to European service stations. The RAC (British) formula for calculating tax horsepower: : :where : ''D'' is the diameter (or bore) of the cylinder in inches : ''n'' is the number of cylinders The distortive effects on engine design were seen to reduce the saleability of British vehicles in export markets.〔 While the system had protected the home market from the import of large engined low priced (because produced in such high volumes) American vehicles 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 1947〔Change In Motor Tax On January 1. ''The Times'', Wednesday, Feb 13, 1946〕 replacing it at first with a tax on cubic capacity itself in turn replaced by a flat tax applying from 1 January 1948.〔The New Car Tax. ''The Times'', Wednesday, Jun 18, 1947〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「tax horsepower」の詳細全文を読む スポンサード リンク
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