How to work out clutch bell and spur gear sizes (Gear Ratio) on the HPI Savage
First decide the number of teeth on the spur gear by the number of teeth on the clutch bell
47t spur 17t bell = 47/17 = Ratio of 2.76
52t spur 16t bell = 52/16 = Ratio of 3.25
If you want more low speed power for off road use then lower the ratio.
If you want more top speed increase the ratio.
However remember that some spur gear/cluth bell combinations are not possible as they don't fit.