Best place to start would be getting a set for the track you plan to race on. Remember: different sets for different tracks. You can d/l some sets here or from
Download NR2003 Setups at SetupSwap.com
If you are driving late model cars like the COT, choose a recently dated file 2007 or newer. Download a few. That'll keep you from getting frustrated with one set that may not seem to work.
Pick a racing set, not a qualifying set. Once you have a set that you can drive relatively well, then you can try modifying it for speed. Start by increasing the tape on the grille. Drive three or four laps at 70F. Continue to add tape and note how hot the engine water gets. Try to keep it less than 220F, around 200F is the best. This will increase your speed quite a bit but the car will be a little tight. Don't add more than 70% tape until you work out other details. General preference is around 60% for super speedways.
Once you are happy with the tape, reduce the spoiler angle two notches. If your set had 70 degrees, reduce it to 68. Do a few laps and note the speeds. Try to stay on the track without hitting the walls. This is important because you lose speed when you touch the walls. Continue to reduce the spoiler until it becomes difficult to stay on the track. Then back it up to a known comfortable number.
The combination of adding tape and reducing spoiler will give you your best initial speed for your driving style. The rest is trimming the car for stability. Since you started with a prepared set from someone else, all the preliminary work was done for you and all you should have to do are minor adjustments like toe-in, track bar and wedge. You may have to add some camber but don't go too crazy initially because it heats up your tires.
If the track has many tight turns, you may need to move some weight forward for stability. When the CG is slightly forward it works like a gyroscope. It helps keep the car straight if it starts to drift. It also helps you recover from spins. Some people move weight to the rear to compensate for the lack of spoiler pressure, but if they happen to be touched by someone they can't recover from a spin.
Okay, something to mention here is that Daytona and Talladega use restrictor plates and fixed spoilers. Nothing you can do about that for multiplayer use. You may be able to cheat the system in off-line races in may of the settings but no sense in that if you are an online player. You won't be able to load the track if you cheat.
There are other game settings that help you with your speed like adjustments to the steering wheel, throttle and brake smoothness, etc. All these are documented in other posts and leagues.
One helpful link:
RACELINE CENTRAL: Ultimate Racing Car Chassis Setup Guide and Tutorial
Hope this helps.