If you go 48DP then you can divide the 32DP tooth numbers by two, and multiply by three. The equivalent to a 46T 32DP is a (46/2 x 3) is a 69T 48DP, so got for the nearest one you can get to that. If it's a 74T then you will need a 30T pinion for 50mm tyres, and a 34T 48DP pinion for 45mm.
When you convert the pinions from 32DP directly you will not get the same ratios because the 48DP spur is not an exact conversion of the 32DP spur. As has been said before, master the calculation in the back of the book and then you are able to work any pinion/spur combo out at any time. Alternatively go to
http://www.rcracing.com/Gear_Ratio_Program_1100.cfm and make yourself a look-up chart for the spur gear of your choice. HTH
