I found this trick on a website and it is very helpful. All you have to do is buy or win a new car. then immediatly give it an oil change and a car wash before tuning it. All the upgrades will be worth more and youll end up with about 10-20% extra horse power depending on the car. This is very useful in winning the harder races in professional leaguge where you would normmally have to realy on skill.