The overhead camshaft engine efficiency can be improved by control of the rotation and phase of the cam shaft rotation. The aim of this paper is to show the improvements that have been made to the cam shaft rotation through the improvements in the timing belt, and in particular the reinforcing cords within the timing belt. The current state of the art of timing belt reinforcement is presented, and an independent study of the fuel efficiency of a modern timing belt compared to the efficiency of a fully optimised gasoline engine with a timing chain. This considers the power losses due to friction as the timing belt (or chain) transmits power from crank to cam, the power losses expressed as vibrations and noise, the variations in speed of cam rotation for different designs of timing belt, and also consistency of cam rotation through the life of the timing belt (or chain). The improvements have been quantified from engine studies of engine efficiency, engine dynamics, engine friction measurements, accelerometer studies for NVH behaviour and from these the benefits in fuel economy and CO2 emissions were quantified.Novel systems that use a timing belt running within the engine immersed in oil are discussed. These have been adopted on two engines to date, with significant interest and development projects under way.Of concern to the motorist are not only fuel efficiency but also the cost and frequency of changing a timing belt. The historical mechanisms will be discussed of the changes in timing belts that control the belt durability. The improvements in reinforcing cord design and performance that contribute to life-of-engine belts will be presented, together with cord and belt developments that increase the durability and reliability of the timing belt even further.