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A tubbed vehicle has had the wheel houses extended towards the inside of the vehicle. This allows the use of wider wheels than stock without widening the body of the car. When this is done, either wheels with a very positive offset (meaning that the hub mounts towards the outside of the wheel) are used, or for more strength, wheels with no offset are used (the hub mounts at the center of the wheel) and the rear axle is shortened so that the wheels do not stick out past the body.

Tubbing is typically used for drag racing, although a lot of people run tubbed-out vehicles on the street for bragging rights. It is easily done on full-frame vehicles, so it is still sometimes done to trucks, but it is typically a very complicated thing to do to a unibody car and as such is a very bad idea unless you know what you are doing. Typically, any unibody car which will have this modification done will also have a full rollcage installed, which will maintain the rigidity of the car that would be lost when tubbing if you are forced to eliminate structural material to provide clearance.