sentences:  The 6T 650 cc Thunderbird, as designed by Turner, and further developed by Jack Wickes, was launched on 20 September 1949, when three models covered 500 miles at 90 mph (800 km at 145 km/h) in a demonstration at Montlhery.Essentially an enlarged tourer version of the Speed Twin, the 6T was designed to satisfy the substantial American export market, and was advertised as capable of a genuine 100 mph (161 km/h).The Thunderbird became a favourite of police forces worldwide.