3.5 in a xxx-cr? Now that's funny, I don't care who you are! J/K
Good luck keeping the front wheels on the ground or slipper pads!
Depending on the track, 12 to a 19 will lay down the power nicely and keep the parts on the car. I run a 5.5 in a ZX-5 and it is scarry fast. Fast as most 1/8 scales.
I just started running a Birdman 19T in the mod class (BK2) and have been placing in the top 3 most nights. It still pulls the front wheels! Nothing like beating your buddy who is running a low turn mod with a 19T!! Not to mention if the longest straight is shorter than 100', he will have a hard time passing you!!