 Upgrading the engine makes the car accelerate faster, helps the top speed of the car by a small amount, and makes it easier for the car to reach higher gears.Upgrading the nitro makes nitro boosts accelerate more strongly and reach higher top speeds.Upgrading the gearbox mainly improves the car's top speed, improves the car's fuel economy, and can greatly improve the car's acceleration starting with the third out of four gearboxes that are available, but gearbox upgrades require an engine that is strong enough to allow shifting into higher gears in order to be effective.