From the Super Mario Wiki
The Mario Car (or Hero Car) is an item that is available for use in New Super Mario Bros. Wii and is the first and only Koopa Clown Car that is usable by the player. The Mario Car is a very small (just a little smaller than Bowser Jr.'s) type of a Koopa Clown Car that Mario, Luigi, Blue Toad, and Yellow Toad get to use. This item is only used in World 6-Airship, and, even then, it is only used to fight Bowser Jr.. The Mario Car highly resembles a usual Clown Car, except that it either has an M, L, or a Blue or Yellow Mushroom logo on it depending on the player. The only known attack as of now that the Mario Car can use is a spin attack similar to that of an airborne Spin Jump and crashing into Bowser Jr. by trying to shove him into the electricity. The player must shake the Wii remote in order to use this attack.
In Mario Party 9, the second Mario Party game for the Wii, a minigame called Bumper Sparks is found, which has players use vehicles very similar to the Mario Car to bump into Bowser Jr. to try to get him to hit the electric barrier, similar to in New Super Mario Bros. Wii. In Pair of Aces, they are equipped with cannons.
In New Super Mario Bros. U, the concept of riding a clown car is reused in the final battle. The player must steal Bowser Jr.'s Clown Car and use a Ground Pound-like attack (the same one Bowser Jr. uses) to defeat Bowser with it.