A custom car is a passenger vehicle

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

 that has been modified in either of the following two ways. First, a custom car may be altered to improve its performance

Engine tuning

Engine tuning is the adjustment, modification or design of internal combustion engines to yield optimal performance, to increase an engine's power output, economy, or durability....

, often by altering or replacing the engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 and transmission

Transmission (mechanics)

A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...

. Second, a custom car may be a personal "styling" statement, making the car look unlike any car as delivered from the factory. Although the two are related, custom cars are distinct from hot rod

Hot rod

Hot rods are typically American cars with large engines modified for linear speed. The origin of the term "hot rod" is unclear. One explanation is that the term is a contraction of "hot roadster," meaning a roadster that was modified for speed. Another possible origin includes modifications to or...

s. The extent of this difference has been the subject of debate among customizers and rodders for decades.