Here's how: I tuned my suspenison to be as low as possible, keeping both front and rear at the same height as this seems to give higher speeds. Also, my style of driving involves a lot of powersliding so I set the steering option thingy towards 'More'. I change the transmission settings to be towards higher top speed for drags and highway circuits, and towards higher acceleration for all other game modes/tracks. Downforce should be 10.0 towards the rear and around 4.5 (or lower) towards the front if you like powersliding. If you want a more stable but slower car, increase the downforce on the front. Adjust the ECU and Turbo to give the most torque and power towards the middle portion of the graph as this is the speed/RPM range you will be in most while racing on circuits. Give slightly higher power and torque towards the end part of the graph and least in the starting part. This will give you high speeds but it kinda kills the acceleration if you crash up. Play with these settings a bit to figure what suits you best. I set the NOS duration to 5 (default is 6) as I think this gives the best balance between duration and power.
These settings suit my style of driving, but they won't necessarily be suitable for you, so play with the settings a bit to find what suits you.