A few tips.......
be sure to have your car set up off line before going online (your profile name offline needs to be the same as your online name)
about set ups: if you run with assists on and no tire wear try using lower tire pressure (16-20), if you don't use assists and have tire wear on (like me) you will want to keep the tires no lower than 23 unless you are on a short track, I tend to add .5 wedge on most setups, I don't like a loose car, If you like a tight car like I do knowing the breaking points is very important, the mistake most people make racing is going into corners too hard or out of control, If you like a looser setup you might want to add a little downforce to keep the car under control entering corners, this will hurt your top speed a little but will allow the car to turn easier, it's basically a trade off
most important thing about online setups are the gear ratios, you want to set the car so that you are red lining right before you break to go into the corner, if you go too hard you blow the motor but if that's the case you probably went into the corner too hot anyway
Here's an example of a good ratio for Atlanta (if I remeber right, it's pretty close) 1st-3.60, 2nd-2.10, 3rd-1.60, 4th-1.02, rear end 3.60, these same gears work pretty good at Texas and Charlotte also, you may need to tweak a little depending on your driving style and whether or not you are blowing motors, if you blow the motor frequently drop 4th gear to 1.00
hope this helps
Hawk