Upon entering a Car Mechanic with a damaged vehicle, the option to repair the vehicle is given. If the car is not damaged, the option to customize the vehicle is given. Rust and dirt are removed upon entering the Vehicle Customization menu.
Each of these have the options Stock or Upgrade, and each cost $100 at the start of the game and only $7 at once all discounts have been unlocked.
Some cars driving around Stilwater have different Body Mods by default, but none of them ever have Performance Mods equipped.
Nitrous - This is used for a short burst of speed, good to use when starting up a car to improve the acceleration. Additionally, it increases collision damage to the point where a hit cripples or immediately destroys a vehicle.
Hydraulics - (Available on 46 Vehicles) Can be used to make the vehicle bounce or rock.
Speed Upgrade - (Only on Demo Derby cars, equipped by default)
Reinforced Bumper - This is used to allow the bumpers to withstand more impact.
Reinforced Frame - This is used to allow the whole frame of the car to withstand more impact.
Kneecappers - When using this, four spikes come out from the car when it is being driven, which can be used to make pedestrians fall to the ground or to burst the tires of cars. Most vehicles can have kneecappers with the exception of four.
Tire Durability - This makes the tires able to withstand more impact, making them less likely to burst.
Because only some options include the option "none", "none" is counted as an option except with Decals, since "none" is always an option. For example, the 2 options for Casket are "Option 1" or "none".
Sometimes, Bumpers is an option, other times, it's Front Bumpers and Rear Bumpers. Bumpers changes both bumpers at the same time.
Handlebars is only an option for the Toad, and not for any Bikes.
The customization screen now contains the non-customizable vehicle
All vehicles can be reinforced. This includes tire reinforcements on vehicles that do not have tires.
Decals, unique artwork and designs of regularly non-customizable vehicles can not always be changed or the color choices may be limited.
The interface may become buggy and certain changes automatically apply themselves when viewed. This occurs with the N-Forcer trim paint but can be fixed by the applying colors separately instead of using the "paint all" option and, specially, the police variant of Kenshin when you customize the layout to the optional configuration of the standard variant, giving it the skeletonized structure (you can enhance it and paint it, but you shouldn't change the layout, if you want to preserve the police variant of the Kenshin).
Functions of Rim Jobs that are not regularly present in Saints Row: The Third are able to be viewed, for instance, the Crusader is able to receive a Hydraulics modification, despite it having no effect and having no presence in the game otherwise.
The Nitrous upgrade can be bought for tanks but doesn't work.
Some menus have an odd arrangement, for instance, the menus for recoloring the Challenger list each form of paint as a sub-category.
Using a new vehicle off the street to customize a non-customizable one in the garage works for many vehicles, but not all of them.
The customization glitch is possible in Saints Row IV, using the same method in Saints Row: The Third. Since any vehicle can be immediately added to the Garage, this makes it slightly easier to perform.
For example, the Assert can be given working Hydraulics.
The Demolition Derby cars have their rusted reinforcements ungraded when entering the car customization menu, but since the Rampage does not have a Trim Color option, the color of those parts cannot be changed.
The Toad has the modifications options of a car, and is the only vehicle to include a Handlebars option.
The Baron is the only car that can have a Wheel Width of 0.
The Zenith's Spoilers are set via the Roofs option.
The Varsity's Bed Mods option is in Body Mods instead of Details.
During "The Fundamentals", the Vehicle Customization prompt does not appear when driving a vehicle into Rim Jobs. If a vehicle is damaged, it is possible to repair it at Rim Jobs, which automatically adds it to the Garage, and then it is possible to enter Rim Jobs on foot, and enter Vehicle Customization from the menu.