Tuning and Repair
Even if you manage to grab a great car, there are means of making it even better for taking on the course. With Tuning and GT Auto you'll be able to increase any car's performance. These can cost quite a bit of money, so save up from B-Spec, License Tests, and Special Events.
Lowering the car's weight is a huge means of making your car faster and improve cornering. There are three levels of Weight Reduction for the Body/Chassis. While you should always start with the first one, the next too are fairly expensive. To save on money, consider both the Window Weight Reduction and Carbon Hood. Not all cars can use these, but for the ones that can they will be great cheap means of making your car accelerate quicker. The Chassis Reinforcement is also good for adding control while cornering. The Aerodynamic Wings sold at GT Auto can also help with downforce. Adjusting this downforce between both front and rear can give you great top speeds. However, experiment with making one of them high and the other low to find the style that appeals to you.
Increasing Speed and Control
There are a huge amount of upgrades to improve Horse Power. The stages of upgrading the engine are good, but can get pricy quickly. Before going on to stage 2, take an ECU Tuning part to further improve the engine. After increasing the engine, upgrade the Intake to the Sports Intake Manifold; all other options aren't worth the money. For Exhaust Parts, buy the more expensive Titanium Racing Exhaust, along with the Sports Exhaust Manifold add-on. The Turbo Kits really depend on the car, where you will need to consider the stand alone RPM that goes along with the kit model. Most cars can't use the Supercharger, but for the ones that can, feel free to install it.
There are other means of greatly improving your cars that don't directly reduce your KG or increase HP. The 6 Speed Close Ratio Transmission is a great means of giving your car better performance. However, older model gears that only have four or five gears will require the Fully Customizable Transmission to reach top speeds. To improve the drivetrain get both a Twin Plate Clutch and Semi Racing Flywheel. The Carbon Drive Shaft is also worth it. The Torque Sensing Center Differential is good for 4 Wheel Drive, but isn't installable on other cars. The Adjustable LSD is good, but requires some playing around with. The main thing to get from the LSD is the increase in Throttle Control, which can increase the power surging from the engines down to the tires. The Height-Adjustable Sports kit is one of the last parts you'll need to make your car great on any track. However, you'll also need to purchase every type of tire, as each course has either restrictions or has better suited terrain.
Repairs and Extra Modification
Your car will need extra cars as it undergoes racing in both A-Spec and B-Spec. Overtime you'll need to fix the ware and tear that comes with any car. Enter GT Auto to start fixing your favorite racer up. The Car Wash doesn't actually help with performance, but will remove possible blemishes your car may have. You may need to change the oil every so often, so pay a few credits when it runs low. Both the engine and chassis will become damaged after so many races, so be sure to repair them when you can. These prices can stack up if you aren't paying attention, so be sure to make repairs before going and buying a new car. Finally, you can really boost the car via Racing Modifications. Not all cars can use this, but for the ones that can it can be a big performance booster.