18" rear to 16" rear on 74 XLCH
 

Its time I need to buy some new rims and not sure if I would need to chage sprocket size with drum brake if I go to a 16 " rear wheel when im currently running an 18".

It depends on your tire size but generally you don't have to change the size of the sprocket.

What's wrong with your current rims? Are they factory aluminum?

They are chrome rims, spoked and are rusting/pitting, so looking for something new. I had just heard that going to a 16 would change gear ratios and have me revving higher.