The only thing that appears to matter on car selection is the 5 speed or 6 speed. Not that either one is faster, just that if you choose to go manual than you will shift more with the 6 speed.

Automatic/manual doesn't seem to matter either.

Horsepower = Top Speed. If you never reach top speed then any tuning points in Horsepower are wasted. Try to optimize this.

Second upgrade to go for is Aerodynamics, you can control the car pretty well regarless of handling.

To initiate a Power Slide: Turn, release the gas, and tap the brake. Downshifting during turns will also help you maintain speed.

By level 27 you should have about 10 points in Horsepower, 10 points in Aerodynamics, and 8 points in handling.

Keep in mind you can move Tuning Points to other categories anytime. If you can't beat a race then try different Tuning setups.

Typically the end segments of the tracks will determine how to tune your car. For example, if the end has a lot of curves go for more handling. If it is a straight line go for Horsepower and Aerodynamics, and don't be afraid to be a dirty driver, ram your rivals into the wall if you have to!