I would put 2800KV on 4S as a little high as well, but the LRP motors are a little underpowered in comparison with the Castle, Neu motors that we have based that hypothesis on, so maybe the LRPs like to run a little higher RPM.
According to the manual, for 2800KV on 4S, you want to run 11/44 gearing on an S8BXe (11T pinion, 44T spur). By my rather rapid (and probably wrong) calculations, the differentials in the S8BXe are at 14/49.
According to my calculations, that comes out at about 48 mph including tyre ballooning (using the same methods and constants as I always do when calculating gearing), which is a touch high. I'd suggest bringing the pinion down to a 10T for 44 mph, or a 48T spur with the 11T pinion (preferred, as the teeth won't wear as quickly).
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