What is the ruling on equipping a car, modifying it, or increasing its power?

It is permissible to do anything that benefits the car and does not cause harm to its owner or to others, for the default rule concerning things is permissibility, based on the Almighty's saying: "He is the One Who created for you all that is on earth." It is prohibited if it involves harm, based on the Prophet's (peace be upon him) saying: "There should be neither harm nor reciprocating harm," and on the legal maxim: "Harm is to be removed."