You will need to edit your .PLR file to change the strength of the AI
Go to your rFactor/User Data/Your Driver Name, look for the PLR file that matches your driver name and open that with notepad or some other text editor.
Click edit, find and type Game Options [enter]
You can them make changes to the following which help get your desired results.
QUICK AI Driver Strength="110" // 100 should approximate real life
AI Power Calibration="7" // Adjustments with AI strength (0=none, or add the following: 1=power, 2=gearing, 4=fuel)
AI Fuel Mult="0.99000" // Additional fuel multiplier for AIs because of their driving style
AI Tire Model="0.40000" // 0.0 = use AI peak slip, 1.0 = use player's dynamic slip, or a blend between the two (can be overrode in TBC with AITireModel)
AI Brake_Power Usage="0.98000" // Fraction of theoretical brake power that AI attempt to use (can be overrode in HDV)
AI Brake_Grip Usage="0.97250" // Fraction of theoretical brake grip that AI attempt to use (can be overrode in HDV)
AI Corner_Grip Usage="0.97000" // Fraction of theoretical cornering grip that AI attempt to use (can be overrode in HDV)
AI Max Load="40000.00000" // Maximum total load to set up theoretical performance tables (can be overrode in HDV)
AI Min Radius="20.00000" // Minimum radius turn to set up theoretical performance tables (can be overrode in HDV)
AI to AI Collision Rate="20" // Detection rate per second (1-40) for AI-to-AI collisions
Autocalibrate AI Mode="0" // When in a test day with 1 AI, AI will attempt to perfect his driving line, and save his knowledge for future use
AI Limiter="1.00000" // Range: 0.0 (no limiting) - 1.0 (limiting used to make racing closer but also make more driver differences on flat-out tracks)
AI Mistakes="0.00000" // a range of (intentional) AI mistakes from 0.0 (none) to 1.0 (sometimes). Anything above 1.0 multiplies the frequency
Before making any changes, back up your PLR file so you can revert back if your game becomes messed up due to any changes you made.