Z: A letter rating for tires to indicate that they are theoretically rated for speeds over 300 kph (186 mph), as in P245/50ZR16. There is no higher rating, but the one lower rating is Y

Zahnradfabrik Friedrichshafen: See ZF.

Z'ed: When a frame is cut, usually in front of the rear wheels, and a section is welded in to raise the axle so large tires can be used at stock body heights, racers say their car has been z'ed [pronounced ZEED, not ZED], referring to the new appearance of the frame.

zener diode: A semiconductor used on British motorcycles for many years as a voltage regulator. When the voltage across the zener diode reached a certain point, the element would begin to conduct current, routing it to ground, thus preventing the battery from overcharging.

zerk fitting: A small valve that allows grease to be added to a ball joint with a grease gun and prevents the grease from leaking out when pressure is placed on the area. Incorrectly pronounced "zert." Also called "grease fitting."

zero-offset steering: A steering system whose geometry has a scrub radius of zero. This configuration minimizes the steering effects produced during acceleration (with front drive) or braking on varying traction surfaces.

ZF: An acronym for "Zahnradfabrik Friedrichshafen," which is a German company that manufactures transmissions and transaxles used in many exotic, high performance, and racing cars.