ive never seen a puch with a 15 tooth front sprocket.
according to an old model book i had, generally the gearing on puchs went like this:
tomos gearing is the weirdest shit ever. puch makes sense.
smaller front = more low end
bigger front = higher speed
of course there are limits, so depending on your rear sprocket size, a bigger front could also result in lower and lower speeds, because your bike might not have enough power to pull super tall gear ratios.