Expected FPS (Directx vs Opengl)

I finally have gotten a copy of NR2003 (loved NR4 back in the day) and have it setup along with many additional tracks (will venture into cars at some point, maybe). I have noticed that my FPS stays anywhere between 45-80 with Directx8 when racing a full field with shadows on but shadows on cars off, pit crews off, and infield objects on "some". Even using a stock track (say Michigan), if i start at the back of the pack I get 50 fps. Resolution has no effect on FPS.

The reason I'm asking is...

Expected FPS (Directx vs Opengl)