Denovo Run-flat tyre, eliminating the need for a spare, though this was not commonly selected and is very unusual on surviving examples. The car's primary competitor on the domestic UK market was the Triumph 2000, also released in October 1963, just one week after the P6. In continental Europe, the Rover 2000 competed in the same sector as the Citroën DS which, like the initial Rover offering, was offered only with a four-cylinder engine – a situation which was resolved in the Rover when the V8 was engineered to fit into the engine bay. The Rover 2000 interior was not as spacious as those of its Triumph and Citroën rivals, especially in the back, where its sculpted two-person rear seat implied that customers wishing to accommodate three in the back of a Rover should opt for the larger and older Rover P5. Series I 2000 The first P6 used a 2.0 L () engine designed specifically for the P6. Although it was announced towards the end of 1963, the car had been in "pilot production" since the beginning of the year, therefore deliveries were able to begin immediately. Original output was in the order of . At the time the engine was unusual in having an overhead camshaft layout. The cylinder head had a perfectly flat surface, and the combustion chambers were cast into the piston crowns (sometimes known as a Heron head). With the rapid construction of motorways around the world, many of which (including those in the United Kingdom in this period) had no upper speed limits, the P6's engine was developed to be efficient, smooth and reliable when driven at high speeds for long periods. It had 'square' dimensions where the cylinder bore and stroke were the same (85.7 mm or 3.4 in) to minimise piston speeds and bearing loads, which was very unusual amongst British saloon cars: these tended to have under-square (long stroke) engines, a lingering legacy of the pre-1947 horsepower tax system and the driving conditions encountered on British roads in the pre-motorway era. Cars that were built until 1966 are referred to as early cars. The Rover 2000 had many detailed differences that changed over the first 3 years of production. Items such as exhaust systems, gear linkages and most visibly the front valance. It had vertical slats and was curved with no extra bumps which gave the car the nick name "sharks