Using LPR-type tyres (like the AKA Cityblocks), you'll want an 11T pinion for most tracks, or a 12T if it's a very large track (or if you feel like it) on 5S.
On 4S, using the same tyres, you'll want either a 14 or 15T (for the same reasons).
The 52-tooth spur would be a better option, as it allows you a larger range of pinions that have more teeth (so wear less).
With the 52, on 18.5V, you'd want a 12/13/14T and on 14.8V, you'd want a 16/17T.
12/13 cover the 11 from earlier - neither is perfect, though I'd go towards the 13 if you want speed, and 12 if you want run time (only slightly more of each to be honest).
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