Going to take the plunge and convert my XB8EC

The stock gearing for any XB8 version is the same. The front and rear ring and pinion gears are 40T/12T for the 3.33:1. The stock spur gear is a 46T. I think the only drivetrain number you would need to adjust is the spur gear to 46T. The 40T gear you are reading about in the description is on the differential, not the main spur. With all other numbers staying the same, the calculator will est. about 44mph. A bit under your first estimate, but certainly not slow! If you want to use the XRAY spur gears, they have several available if you need to adjust your ratio, from 44T-48T.