Tax horsepower

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.