What tyres and downforce settings are you using?? Tyres should be Racing SOFT.
Don't give the AI cars your slipstream, use their slipstream when you are behind them but once your in the lead make sure you don't give them your slipstream.
Settings also play a big part, check the tuning sub section of ths forum for a full setup.
Quick tip, go to settings> (for your RM ZR1), suspension> (click on this) look at the figure on the right hand side called ''TOE'', if your REAR TOE is +0.20, change this to -0.20, your car will turn a bit better, also, have medium / high downforce on the front, rear downforce at minimum. Plus, make sure your gearing is correct i.e. you're not hitting the rev limiter, but you are getting full revs.
This is for Indy, not Daytona Road, you'll need different setup for that.