The reason the Austin 10 is called a 10 is due to a now obsolete horsepower rating system from the RAC (Royal Automobile Club). Most early cars included the R.A.C rating in their description and it became the accepted method for determining the class of car. Surprisingly the RAC horsepower rating of car engines was given in the technical specifications of a car up until the 1960's, despite the figure having little connection with an engine's actual power by this time.
The RAC rating was very important, as the car tax was based on this rating. The rating was a simplified way of calculating engine brake horsepower in the early years of the motor car. A number of assumptions were made by the RAC about engine design to simplify the calculation, however engine development soon meant that the rating became less of an indication of an engines real performance and merely represented an indication of tax group. Already by 1932 at the launch of the Austin 10 the actual power of the engine was closer to 20hp than the RAC quoted 10hp.
The primary reason for the discrepancy is that three factors in engine design were assumed to remain constant.
Mechanical efficiency of 75%
The mean effective pressure of 90lb/sq.in
The mean piston speed of 1000 ft/min.