Replies sorted oldest to newest
What width rims are on the car? Is there fender clearance for a wider tire on the back? How much shorter do you want to go? You can run a shorter tire on the front than the back; remember that the shorter the front tire, the more out your speedometer will be.
Slammed - looks like you may have the standard 4.5" wide wheel (like a LOT of us on here). On my previous VS, with that set-up I was able to run 185x65x15 without any clearance issues (which would usually be at the rear wheels). Lots of clearance room up from, even with those drop spindles (typically) widening your track by 3/4 inch. 
You could go with a 175/55-15.
http://1010tires.com/Tires/Kumho/Solus+KH16
165/80-15 diameter is 25.39 inches
175/55-15 diameter is 22.58 inches
....or if you really want to go low you could go with a 165/45-15
http://www.directbuytire.com/product-p/176k5afa.htm
165/45-15 diameter is 20.80 inches 
Personally, I wouldn't go wider than 175 mm with a 4.5" rim.
In my opinion, these cars look right when they are level (front to back, measure from the deco strip under the door or body on the side in front of and behind the door) or ever so slightly lower in the front. The tires have to more or less fill the front wheelwell openings for aesthetics. With tires much shorter than 23 1/2 or 24" tall and you have to lower the car a fair bit for it to look right, so you have to decide what ride height to go for. If you're trying for that "just off the pavement" look (and the car is not air bagged), this may limit where you can go. If you want to be able to jump in it at a moment's notice and drive any where, you may have to compromise, as all it takes is one good smack on the exhaust (or bottom of the engine) and now the thing is louder than crap or leaving it's life-blood on the roadside.
As Ron said, you're somewhat limited for tire selection by the 4 1/2" rims. You can put 185/70's on them (but they're quite tall), and I have heard of guys using shorter 185's. It works as these cars aren't really that heavy, but it's not recommended. Ron's advice to go with a 175 is probably best; just choose the profile/tire height you want. Al
