1932 Ford hot rod. A custom car is a passenger vehicle that has been modified in either of the following two ways. First, a custom car may be altered to improve its performance, often by altering or replacing the engine and transmission. 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 rods. The extent of this difference has been the subject of debate among customizers and rodders for decades. Additionally, a street rod can be considered a custom.