Tuning in Carbon is pretty much broken, so some info here won't make sense in a real car. Thanks to jeffareid and Street_king for their guides on gamepressure.com forums.

Here is a docsheet with parts and car compare (still WIP): https://docs.google.com/spreadsheets/d/16tCI7BG3SwXcZSX2USkNb_LHTUZD9w_IZai3f4RxfZQ

1 means left slide, 2 is middle, 3 is right slide

Engine - The most important part of all. It improves your times by a lot, so it should be a top priority to get this part as soon as possible. Setting is always 333.

Transmission - I found no difference in acceleration or lap times compared to stock transmission. The only difference is top speed, which you'll hardly use. Some transmissions even make the car slower, like with Elise. Stock is just fine.

Suspension - Proper setting can make your car faster in corners, however for some reason suspension makes your car accelerate SLOWER. Fortunately real lap times show basically no difference apart from more comfortable handling, so if you have suspension available, use it. Canyons are always faster with suspension since it's downhill, unless you're driving a car that doesn't need setup, like Lancer. Note that Oversteer and Understeer are mislabeled on the PC version.

Setup for each car:

350Z - 2 2 2 Alfa Romeo Brera - 3 2 2 Aston Martin DB9 - 3 2 2 Camaro SS - 1 2 2 Charger R/T - 1 2 1 Carerra GT - 2 3 3 Challenger Concept - 1 3 1 Charger SRT8 - 1 3 1 Chrysler 300C SRT8 - 1 3 1 Eclipse GT - 2 2 2 Ford GT - 3 2 2 Gallardo - 3 2 2 Impreza WRX STI - 2 2 2 Jaguar XK - 3 2 2 Lancer - 2 2 2 Lotus Elise - 3 3 3 Mazda Speed3 - 3 2 3 Mazda RX7 - 3 2 2 Mazda RX-8 - 2 2 2 Mercedes AMG - 3 2 3 Mercedes CLK 500 - 3 2 2 Mercedes SLR - 3 2 2 Murcielago - 2 3 3 Mustang GT - 1 3 1 Mustang GT 500 - 1 3 1 Nissan 240SX - 3 3 3 Plymouth Barracuda - 1 3 1 Porsche 911 - 2 3 2 Porsce Cayman S - 2 3 3 Renault Clio V6 - 2 2 2 Skyline - 2 2 3 Supra - 2 2 2 Vauxhall Monaro VXR - 1 3 1 Viper - 1 3 1 VW Golf R32 - 3 3 3 Z06 - 1 3 1

Nitrous - Always 111. The difference between velocity and duration is actually quite big.

Tires - Tires make a difference, although not a big one. Common setting is 2, but if your car understeers a lot, go for 1. If it oversteers, go for 3.

Brakes - Stock brakes are already very strong, so there is no reason to use any upgrades.

Turbo/Supercharger - There is very little difference in acceleration with turbo/supercharger, so upgrade it only if you also upgrade more important parts.