Try using the patch which allows rFactor to use much more RAM. Should be able to easily find it. I believe it's called 4GB patch or something (can be used in many games, not just RF1).
In the rFactor display settings, do the following:
- Lower your max visible vehicles
- Lower anisotropic filtering by 1 click (makes a difference with lots of cars).
- Disable shadow blur
- Lower opponent detail to medium
- Lower shadows to high
In your player-PLR file, set garage LOD or garage detail, or something like that, to a number like 0.2. I believe default is around 0.6. This will help reduce those big framerate drops you get when there are many opponents and you're on track driving down the straight next to the pits.