All tuning parts have something in common: the slides that set the tuning.
These slides are measured in percentages, ranging from -100 to +100 with 0% as the stock "factory" setting. This stock setting is usually the best setting to get the average (comparable) times when compared to other vehicles in the same class.
Each tuning option is unlocked when you level up the car and has three (or four if you count no tuning) stages:
Stage 1: ranging from -100 to +100 with 0% as the only option in the middle. this is for the "raw" tuning and has little use to get the most out of your car.
Stage 2: this adds the option to finetune it all a little more. It gives the options -50 and +50.
Stage 3: full tuning. This changes the slides to segments of 10% making you able to finetune what you already had. This should gove you the edge over your opponents when done correctly.