New wheels...

The only disadvantage is that you need to buy new tires Actually, that can be a good thing. Rim diameter does not really mater much, except that you need lower profile tires with 18's over 17's. Lower profile tire "usually" give a little rougher ride, but are better for handling. Traction should be the same between 17's and 18's.

Width is the 8" or 9" you that goes with teh 17x8 or 18x9. Wider is better for traction, but bad for following the ruts in the road. Wider rims and tires can also effect driving in the rain. Usually wider (9" or 10") in the rear is good for traction, and a narrower wheel like 8" is better in the front. Having a little wider wheel in the fornt can help with turning while racing or in the twisties, but will cause the car to track the ruts in the road.

Offset is measured in millimeters from the oustide of the wheel to
where it rest on the wheel hub. Backspacing is usually measured in inches and is the opposite of offset. What offset you run depends on a few things: rim width, front or back, and what you think looks good. I have 18x8's up front with a different offset then stock (less offset) so they stick out a little further to fil up the wheel wells. Same in the rear, the rear has a tad less offset then a standard 9" wide rear to push the wheels out a little further for better looks. I don't remember off hand what they are, but if I remember I wil post it.

Does that help?

Just also wanted to note that its not the rim width that matters for traction, but just the tire width. Of course, they are relative.... If you get a 10.5 inch rim though, and put 275 tire on it, you won't have as much traction as if you put a 305 on it.

So if you get 18x9 or 18x10.5, get a nice wide tire for it.