This guide is for PC platform only. PS could be same, but I've never played it, so I don't know.

Engine: +100% - More power in high rpm is exactly what we want. Brake balance: -100% - Tooltip says you should set this to +100% (Rear) in order to drift, but it's actually the opposite. You always want -100%. The only exception is RR with its sharp hairpin shortcut, where -100% makes the car spin if you use tuning and traction, so use +100% instead. Steering speed: +100% - This is pretty much pointless if you use instant steering trick, but if you don't, you need +100%. Gearbox ratio: -100% - Stock top speed of CLK-GTR is 324kph and you can tune it up to 387kph. Only RP and SU use -60% due to the track layout and 5th gear skip. Aerodynamics: +100% - Since NFSIII has tracks with a lot of jumps, you always want to keep the "flight-time" to minimum. Plus the car corners a bit better. It seems like only Hometown benefits from less aerodynamics, otherwise keep it +100%. Suspension: +100% - Soft settings clearly makes the car steer better, but you loose a lot of speed during cornering, that's why stiff settings is better, because you can keep more speed. However -100% is better if you play with gofast. Tyres: +100% - Best setting if you drive without weather, otherwise use -100%