dashboard, with instruments ahead of the driver rather than over the centre console, initially finished in black plastic and then from 1973 finished in wood. The engine was now rated at 63 horsepower for the UK market, with a 9:1 compression ratio and twin SU HS2 carburetors. (The less powerful North American version continued to use a single Zenith Stromberg carburetor and an 8.5:1 compression ratio) due to the German DIN system; the output was the same for the early Mark IV. Performance was slower than the Mark III due to its weight increase and taller 3.89:1 final drive as opposed to the earlier 4.11:1. The Mk. IV engine displaced throughout the production run, and in 1973 received larger big-end bearings to rationalise production with the TR6 2.5-litre engines. The engine was also detuned to meet new emissions regulations. With the overall weight also increasing to performance dropped, with now in 15.8 seconds and top speed reduced to . Fuel economy was . The gearbox gained synchromesh on its bottom gear. A revised hardtop also became available, with rear quarter-lights and a flatter rear screen. Importantly, the heavily criticised rear suspension was decambered, incorporating what Triumph called a "swing spring". One leaf of the suspension "stack" was eliminated and only the bottom leaf was attached rigidly to the differential. The remaining leaves were mounted to pivot freely — eliminating the worst characteristics of the original swing-axle. This was a different approach than that taken with the Triumph GT6 Mk II (GT6+) and Triumph Vitesse Mark 2, both of which received new lower wishbones and Rotoflex half-shaft couplings. The result on all these cars was improved handling. The Mark IV went on sale in the UK at the end of 1970 with a base price of £735. Spitfire 1500 (1974–80) In 1973 in the United States and Canada, and 1975 in the rest of the world, the 1500 engine was used on the MK IV body to make the Spitfire 1500. Although in this final incarnation the engine was rather rough and more prone to failure than the earlier units, torque was greatly increased by increasing the cylinder stroke to , which made it much more drivable in traffic. While the rest of the world saw 1500s with a compression ratio of 8.0:1, the American market model was fitted with a single Zenith-Stromberg carburettor and a compression ratio reduced to 7.5:1