The 1,147 cc engine was replaced with a bored-out 1,296 cc unit (the bore increasing from to , stroke retained at ), as fitted on the new Triumph Herald 13/60 and Triumph 1300 saloons. A new quieter exhaust gave a sweet distinct note and reduced cabin noise. In SU twin-carburettor form, the engine put out a claimed at 6,000 rpm, and of torque at 4,000 rpm, and made the Mark III a comparatively quick car by the standards of the day. Options included wire wheels, factory hard top and a Laycock de Normanville overdrive. The Mark III was the fastest Spitfire yet, achieving in 13.4 seconds, and reaching a top speed of . Average fuel consumption was 33mpg. The Mark III continued production into 1971, well after introduction of the Mark IV.