Crashday lets players customize their vehicles to a great extent. While most of the tuning kits are visual-only, certain kits like suspension affect the vehicle's performance.
Available tuning kits differ from vehicle to vehicle. Urban cars such as Cube offer the most available tuning kits for a vehicle.
Available tuning kits
- Performance kit: Performance kits usually increase the acceleration, top speed and class of the car and reduce its braking distance and weight. Visual upgrades include a louder transmission, turbo sound and sports steering wheels. Certain cars receive exclusive visual upgrades such as additional exhaust pipes like Ironhorze.
- Crashpower kit: Increases the wrecking power of the vehicle. The weight of a vehicle affects the wrecking power, heavier cars usually deal more damage than lighter cars.
- Armor kit: Increases the vehicle's resistance to impacts such as gunfire.
- Sports: Great road traction but poor offroad performance.
- Super Sport: Largely the same as Sport wheels, the only difference is a wider range of available wheels.
- Offroad: Slightly worse road performance in expense of a much better offroad performance.
- Sports: Lowers the ride height, stiffens the springs, increases damping and aligns the wheels accordingly.
- Extreme Sports (only for Cube, Hunter and Buster): Nearly the same as sports, however the changes are more pronounced now.
- Custom (only for Ironhorze and Firespitter): A different wheel alignment than sports suspension, as well as stiffer springs. This suspension slightly increases the acceleration.
- Offroad: Softer springs and an increased ride height makes this suspension ideal for offroading.
- Offroad Monster (only for Pickup and Wrecker): More or less the same as offroad suspension, the ride height is even higher.
- Front Bumper
- Rear Bumper
- Rear Wing
- The ability to customize the license plate was cut. 
- Flip-flop paint was cut. 
- Missile capacity of a vehicle could have been increased with performance kits. 
- 'Jet engine' performance upgrade can be seen in 2003 garage screenshots.
- A vinyl for the Judge was cut.
- A vinyl named 'Impact Turbo' for the Hunter was cut.