x: Originally this letter indicated "experimental" vehicles, but later came to indicate an "exotic" or unusual vehicle.

x engine: See X-type engine.

x-engine: See X-type engine.

x tire: Michelin "X" Radial tires.

x-type engine: An engine in which the cylinders occur in four rows set at an angle from each other with the crankshaft running through the intersection of the X. The single crankshaft is turned by all banks of cylinders.