A semi-automatic gearbox is an automatic box with manual override. The on-board computer systems combine data from the engine, transmission and gear lever to control the throttle, clutch and gearshift during each change. The driver stays on the throttle during the shift and merely pulls the lever to go up a gear or pushes it to go down, the clutch pedal is not needed except for pulling away from rest. But these types of transmission come in various forms: One of them is the 'Pre-selector' Transmission that the older drivers will probably remember being fitted to the Lanchester cars of the forties. These had a fluid flywheel that was engaged by dipping the clutch very swiftly. From then on it was like driving a normal automatic car. Citroen developed one in the 1970's for the GS range. It was roughly the same as driving an automatic car other than you had to select the gears the same as you would with a manual car but without the clutch pedal. Constantly Variable Transmission (CVT) was introduced by DAF as the 'Variomatic' transmission. It has recently been re-introduced. The principal behind this type of transmission is one of being an automatic but with no real gear selection capabilities. It it enables the car to virtually 'free-wheel' at higher speeds therefore, saving fuel.The CVT is now available in some of the smaller Ford cars. This type of transmission is ideal for town driving and short 'stop-start' trips more so than for motorway journeys. Porsche introduced the Tiptronic system in 1990. Based on an automatic transmission with torque converter, besides conventional auto mode it offers a manual override allowing the driver to shift by pushing the shifter forward and backward. It is no quicker than a conventional auto, it only intends to enhance driving fun through involving the driver. The system is also used in various Audis, Volkswagens, Honda's and some other cars.