[BrianG]

Increase spur tooth count=lower gearing, and vice-versa
Increase pinion tooth count=higher gearing, and vice-versa.

Easier to just divide the spur by the pinion to get a number. The lower the number, the taller or higher the gearing is. And, the higher the number, the lower the gearing is. In your setup 48t/15t=3.2:1, which means the pinion has to turn 3.2 times for every spur revolution.

And, for a typical setup, going up or down by one pinion tooth creates about the same ratio as three spur teeth. Example:
Your combo: 48T/15T=3.2
Original pinion, but increase spur by 3T: 51T/15T=3.4
Original spur, but decrease pinion by 1T: 48T/16T=3.428