Super drive

The super drive was a hyperspeed-enabling turbo mechanism installed in Bessie by the Third Doctor. Although his vintage roadster model was generally used and suited for leisurely drives, this special function was designed for the purpose of urgency. It gave Bessie enough of a momentum boost to easily reach extreme speeds, be it for immediate getaways or rapid transit under a deadline. The trigger mechnaism for the super drive was a sliding lever positioned near Bessie's anti-thief device, and by tugging it, Bessie would then launch into breakneck speeds.

History
The Doctor first used the super drive when he and Jo Grant needed to reach the village of Wootton from a distance of 72.9 miles before 2 o'clock, when would perform an experiment to release Kronos. The Doctor decided to activate Bessie's super drive and flung the car into outrageously high speeds. Jo insisted that it was dangerous for the Doctor to drive at the dangerous velocity he was going, but he assured her that his reactions were ten times faster than hers (though this may have been only a boast), then hit the brakes on Bessie to demonstrate how well it could absorb inertia, whereas a normal car would have sent them through the windshield. (TV: The Time Monster)