The Spitfire racing programme was successful,[1] and in 1965 resulted in 13th overall and a 1st in class at the prestigious 24 Hours of Le Mans (beating their main rivals, the MG Midgets).

The car was further developed and refined and eventually launched as the Triumph GT6 (dropping the "Spitfire" prefix) to emphasise its GT styling and its 6-cylinder engine.

The new body was a sleek fastback design with an opening rear hatch, earning the GT6 the nickname of poor man's E-Type.

[1] The interior of the GT6 features a wooden dashboard housing a full complement of instruments, with carpets and heater included as standard.

Moreover, the unit was comparatively smooth and tractable, in contrast to the MG's four cylinder BMC B-series engine, which produced almost the same power in twin carburettor form, but at higher revs.

[citation needed] The only major criticism was of its rear suspension; the GT6 inherited the swing-axle system from the Spitfire, which in turn was copied from the Herald small saloon.

Triumph had done nothing to improve the system for the GT6 and the tendency to break away if the driver lifted off the power mid-corner was not helped at all by the increased weight at the front of the car.

Triumph realised that they needed to find an answer to the handling problem, if only to maintain their reputation in the U.S. Their response came with the 1969 model year, with the introduction of the GT6 Mk II, branded in North America as the GT6+.

The rear suspension was significantly re-engineered using reversed lower wishbones and Rotoflex driveshaft couplings, taming the handling and turning the Triumph into an MGB beater.

Under the bonnet, the engine was the uprated Vitesse MkII unit developing 104 hp (78 kW) with a new cylinder head, camshaft, and manifolds.

The early emission systems consisted of a diaphragm type EGR valve, carbon canister and evaporation lines and were used on other cars of the period.

[1] Racing mag or aluminium wheels, custom chrome plated steel roof rack, bumper guards, space for under dash radio were also listed.

[1] The last USA models performed relatively poorly, owing to the compression ratios being reduced to allow the use of lower octane unleaded petrol.

The Mk III never sold in the numbers hoped for by Triumph, and was comprehensively beaten in the marketplace by the MGB.