A mathematical formula historically used by taxation authorities to classify cars into tax brackets based on engine dimensions, such as cylinder bore and number of cylinders, rather than actual brake horsepower. This system heavily influenced engine design in certain regions, prompting manufacturers to build long-stroke, narrow-bore engines to keep the tax burden low for buyers.
Heard on the job
That old British roadster has a tiny bore to keep the tax horsepower down, which is why it struggles on the highway.
Also heard as
- taxable horsepower
- RAC rating