I tune cars for maximum lap times, so i can offer you tips in those areas and also a few in others if you want them. the first thing is to press "Y" for help. it will tell you what each adjustment does. otherwise, here are some basic nuggets of awesomeness:
tune fast cars for mild understeer: this prevents the car from sliding when you turn at high speeds and also makes it more stable. you can do this by adjusting the front anti-roll and spring stiffness with relation to the rear. you can also mess with tire pressure and camber.
tune for cornering: you might not want to give up speed by increasing downforce but it is worth it in the long run by allowing you to corner faster.
don't let your car loose grip: adjusting the camber, tire pressure, suspension stiffness, and anti-roll stiffness can improve your car's traction under load.
is there a specific situation you are trying to tune for?
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Subject: tuning tips? 
