The SM 1500 Roadster
The addition of a second carburetor produced a substantial boost in power and performance, increasing engine output from 48 h.p. for the 4AD to 58 h.p. for the SM 1500. This also provided an extra 5-6 m.p.h. in top gear and surprisingly had little impact on gas mileage.
The gear ratios remained unchanged, which prompted The Autocar to write " the impression was gained that a higher top gear would be acceptable, not with the idea of increasing maximum speed, but of lowering the engine speed when cruising fast".
Unfortunately, the specialty niche that SM 1500 had fallen into by this time seemed to isolate it more and more from the market. Buyers were replacing their open cars with closed Saloons and the Roadster was increasingly equated with the out and out sports cars. This meant that it had to compete for market share with the Triumph TR-2, the MG - TF and the Sunbeam Alpine. With an outmoded design and financial difficulties, the writing was on the wall. Only 189 of these cars were built in the final year of production prior to the takeover of Singer by Rootes in 1956.