Try fitting a Kimbrough 66t spur, using the Traxxas slipper pucks in the holes where the bearings would go. This should get you to the 5 FDR your looking for pretty easily.
The 17.5 loves tall gearing and doesnt produce much heat, it is no comparison with the 5700 mamba. I ran the 4600 mamba in my ZX5 running 21/81 and it was awesome.
Let me know how the experiment works, I have thinking about running a 13.5 or 17.5 in my 4wd cars, I just drive better with the lower RPM motors.
--Joey --Venom Batteries & Chargers!!
Old School Racer & Vintage RC Car nut
RC10CE (X4),B2,B3,B4,B5, B5m, B6D - 10T,T2,T3,T4,T5M - B44.3
BRZ-444, RC10M, RC10L3 oval, L4 oval X 2