I've already said earlier that tuning options in "Most Wanted" aren't too complicated. The game is less "demanding" than the products from the "Underground" series, HOWEVER you mustn't forget about buying new performance upgrades. Otherwise you would have some serious problems with most of the challenges. Thankfully, you won't have to buy every possible upgrade, especially at the beginning of the Career mode. Better parts will be unlocked after beating drivers from the Black List. Performance upgrades are the most important elements of the vehicle tuning. Here's what you can buy:
Engine: Bear in mind that the list of the unlocked packages depends on how many drivers you were able to defeat. You should also know that some cars will only allow you to choose between two sets of parts (not three). I'd personally recommend avoiding any contact with these type of vehicles, because you'd have some serious problems with the early challenges. These type of modifications affect acceleration and top speed.
Gearbox: Gearbox modifications affect two bars. You will be able to notice a slight increase in acceleration, however the more important factor is the top speed. As a result, you should be interested in these packages if you have one of the slower cars.
Suspension: The answer is a rather obvious one, at least in this case. You will be able handling abilities of the current vehicle. Furthermore, the most expensive suspension upgrades will also allow a minimal increase in acceleration and top speed.
Nitro: You should already know something about these kind of parts, especially from the previous Chapter of this game guide. You will be able to choose between several different types of instalations. Buying Nitro upgrades will affect your acceleration, however you will also be able to increase an overall top speed.
Tires: The results will be similar to the Suspension upgrades. Buying better tires will affect your handling. Furthermore, you will be able to notice a very high increase of this value. As a result, you should buy new tire sets as soon as possible.
Brakes: I think it's rather obvious that this modification will affect your handling abilities. While the increase seems to be very slight, you should notice the difference during the race itself. There are many challenging corners in the game. You should have the best possible brake sets in order to decrease a possibility of hitting the nearby walls.
Turbocharger: You should know what to expect from these kinds of parts, especially when you've taken a closer look at the Performance Tuning window. As for the bars, you will notice a slight increase in acceleration and top speed.
Two other windows aren't that important. You will have an opportunity to improve the visuals of your current car. The first group contains full body-kits. The list of possible upgrades depends on the chosen car. Some vehicles have only 2-3 body-kits, while the others may be rewarded with 4 or even 5 different packages. You will also have an opportunity to buy other groups of parts - rear spoilers, new rims or the improved hoods. Bear in mind that new parts will be unlocked after taking down drivers from the Black List. Third menu will allow you to paint your vehicle or its rims (there are three types of paint available), buy additional vinyls (also unique ones), apply window tint or acquire sport gauges.