Re: Horsepower (was Re: Metric Time (was Re: Why not 13 months? (Was

I Johnston (
17 Oct 1995 16:23:58 GMT

Jacques Guy ( wrote:

: Much later, I read (I think it was in New Scientist), that
: a horse has a power closer to 12hp, early automotive
: manufacturers having deliberately grossly underrated a
: horse's strength to sell their wares.

A confusion between two distinct fiddles:

1) The power of a horse was originally measured by James Watt as 22,000 foot
pounds per hour. However, he defined the horsepower as 33,000 so that
mine owners who bought a 4hp steam engine (say) would be impressed at
how easily it did the work of four horses. Thus was the horsepower

2) Taxation of steam and later internal combustion road vehicles in
Britain was for years based on engine power. To reduce tax,
manufacturers came up with the "nominal horsepower" based solely on
engine dimensions and models of power output which were grossly out
of date and far too low. Thus was the horsepower underestimated.

Example: A 1920 motor car might have a 20hp engine, equivalent to 30
horses and be taxed for a nominal horsepower of 4nhp.

When taxation changed manufacturers soon decided that playing down the
power of a car wasn't going to make it sell any faster....