With the UE diffs, an approximation would be to add 4 teeth to the pinion.

I want to make sure I am thinking clearly. UEdiffs mean I need larger pinion +4. Now lets throw in the 51 tooth spur. Now what am I back down to? Aaarrrggh
:P: !

I think the ue DIFFS require adding two pinion teeth, not 4! The Ofna 51 tooth spur gear changes things again! And yes, I have 12 volt and 5 volt fans that would work in these applications.