The RAC HP Rating

All pre-war vehicles have an RAC horsepower rating, which was linked to the Vehicle Excise Duty or VED, aka Road Tax, Car Tax, Tax Disc and Road Fund Licence, and a figure for brake horse power.

When the horsepower tax was introduced on 1 January 1910 a formula, known as the RAC Rating, was used to calculate the horsepower 1.  The formula was based on applied theory and derived from basic principles rather than from the results of a bench test.  It had three assumptions which never varied: a mean effective cylinder pressure of 90 psi; a mechanical efficiency of 75% and a mean piston speed of 1000 ft/min.

hp = ( D2 x n ) / 2.5

where D = the diameter of the cylinder in inches and n = the number of cylinders. 

The VED payable was: 0 to 6.5hp – 2 guineas (£2.10); <12hp – 3 guineas (£3.15); <16hp – 4 guineas (£4.20); <26hp – 6 guineas (£6.30); <33hp – 8 guineas (£8.40); <40hp – 10 guineas (£10.50); <60hp – 21 guineas (£22.05).

The rate was changed to £1per hp on 1 January 1921, encouraging an interest in light cars and effectively limited large engines to luxury cars; ideal for the Austin Seven to have a low VED.  With RAC Ratings of 7.8hp and 7.99hp rounded up, perhaps the ‘Seven’ should have been called the ‘Eight’ 



1923 -1929

1929 - 1936


1937 - 1939






Bore – ins,





RAC Rating - hp





Output - bhp







R J Wyatt writes 2 that in 1924 “Sir Herbert Austin proposed a new formula for taxation which took into account the cylinder swept volume, rather than bore size only, which penalised engine with a high stroke-to-bore-ration.  His system was simple.  He took the swept volume of the cylinders, in cubic inches, and divided by 10, this figure would be the unit of taxation, giving the Austin 20 a rating of 22 against the RAC 23; the 12 would be 10 against 13; and the 7 would become the car to gain most from his proposal dropping from an RAC 8 to an Austin 4, halving the annual tax to £4.  There is no doubt that the method was more logical, but in the light of a decline in annual average tax and an increase in the number of cars that would benefit most, it is not surprising that the Minister of Transport declared that he would not consider any scheme which reduced revenue, and that re-imposition of petrol tax was out of the question.”

By 1930, engine developments meant that the effective cylinder pressure was 125 psi, and piston speed had doubled.  Some manufacturers, such as Rover and Wolseley, started to use a double rating system and designated a car as eg. 10/25, where the 10 was RAC horsepower and 25 was indicated horsepower 1.   If Austin had used this nomenclature, with the 8hp RAC Rating, our cars would have been the Austin 8/10; the Austin 8/13; then the Austin 8/17 and the ‘Big Seven’ as the Austin 8/25 - which do not sound so passionate as ‘Chummy’ or ‘Sevens’.

The RAC Rating was used until 1947 and replaced by a flat rate of £10 per year for all cars.

1. For an explanation of the derivation of the RAC formula look at ‘The RAC HP (horsepower) Rating - Was there any technical basis?’  by Richard Hodgson at

2. R J Wyatt. 1981. The Austin 1905-1952 pub. David & Charles.

This article, written by Doug Castle, originally appeared in CA7C Seven Focus in May 2009 pp24-25.