157dB

I went from 16s to 17s to 18s.
255/45-18s the first time around
and 275/40-18s this time.

Wendlandt

I've posted here before regarding different wheel/tire choices for my '06 GT and didn't have any luck with specifics. I have 18s now and would like to go to a smaller wheel also for the extra tire sidewall...both for looks and ride. What can I do??? I will be buying wheels and tires this winter and install next spring so I have time to research, order, etc.
(and save up the money!)

157dB

17" is about the smallest you can go with the GT front brake package you have.
Be very sure the rims you choose fit the calipers before installing tires on them.
The correct offset will also keep your scrub radius to a minimum.