MM5700 in Losi XXX-T

I'm running a MM 5700 in my Losi XXX-t and have it geared 20-pinion and 86-spur.

Does this sound right ? I see some guys saying around 14-15 for the pinion and 86 for the spur (this is probably for buggy's but would it also be for trucks ?). Wouldn't this slow down the car (If 'im correct when you gear down the ratio goes down meaning a higher number).

I also see messages about BEC/UBEC !!!??? Is this something I need on top of the MM ESC and Motor ?

Thanks