Engines: the core of the car, each engine has three different attributes (Speed, Accel, Turn) whose values vary between +70 / 100. Every NFT can be leveled to boost its attributes, this can be made by locking the engine in the Pit Stop Contract and paying a fee for the boost.
Chassis: attributes given by the chassis are the same as the engine but instead of giving a fixed value, these values are percentages (0% / 10%) that are applied to the engine's attributes.
Tires: last but not least, there are different types of tires that are needed to improve your time on each circuit. As every circuit is different (number of turns, different turn degrees, straights length, weather and road type) you may want to switch tires for every track.