[Lil' Lavery]

Your clearance over the ramp will depend greatly on your wheel placement, not only the size of your wheels. Robots who have their wheels placed near the edges of the 38" space will need greater ground clearance than those who are mounted further inwards. The reasoning to space them further out being a more stable drive base in terms of center of mass while climbing the ramp (although less stable if taking impacts from the sides). Additionally it matters how your wheels are mounted to your frame (how far beyond the bottom they extend, as it really isn't the size of the wheel that matters, it the ground clearance, and the two factors are not the same).
Additionally, the relationship between ground clearance and wheel size is altered by the number of wheels you have (as this will change the contact points as you travel up the ramp).
Also, as mentioned before, a larger wheel results in less rpm on the drive shaft to acheive the same speed, which in turn means less stress on the bearings.