I'm glad I asked.... some good info

 

The advantage of 18" wheels over other sizes? For what purpose?

If for solely street driving then it would be cosmetic (looks cool).

If for car shows then larger wheels look impressive and unusual not to mention it costs more so it would be more exclusive. 19" or 20" might be more attractive.

If for performance driving then you have to assess the relative advantage of 17" vs 18" or 19". larger diameter usually means heavier weight, options for wider rims and less available designs for 4x100 bolt pattern.
This means when looking at tirerack.com, edgeracing.com or 18racing.com we find many more 17x7 or 17x7.5" rims to choose from in 4x100.

For a 17" rim you can run 40 or 45 series tires easily and selection of tires is relatively good until you look at track tires then 225/45-17 seems the limit. Stock tire diameter is about 24.4"

For 18" rims (with limit of about 25" tire diameter) you can run
205/40-18 fits rims 7-8" wide outer tire diameter 24.5"
215/35-18 fits rims 7-8.5" wide outer tire diameter 23.9"
215/40-18 fits rims 7-8.5" wide outer tire diameter 24.9"
225/35-18 fits rims 7.5-9" wide outer tire diameter 24.4"
235/30-18 fits rims 7-8.5" wide outer tire diameter 23.6"
235/35-18 fits rims 8-9.5" wide outer tire diameter 24.5"
Most tires weigh around 20-23 pounds.

Tirerack.com has quite a few 18" rims at decent prices until you look at the weights then the lighter ones are more expensive. A heavy 18" rim with 35 series tire will not take lightly to hitting a big pothole so check your roads before you buy and drive carefully.

For 19" rims (none at tirerack.com)
215/35-19 fits rims 7-8.5" wide outer tire diameter 24.9"

For 20" rims
225/30-20 fits rims 8-9" wide outer tire diameter 25.5" (coilovers!)
BF Goodrich g-Force T/A KDW 2 costs $258 each!

Another consideration is which tire is best for your purpose- for street tires you can find something in many of these rim sizes but for competition and R-compound tires some like the Kumho V710 come in very limited sizes.

Finally, big brake fits won't fit smaller rims very well compared to at least 17 or 18" rims and the proportions of rim to brake rotor and caliper size is better with larger brake and larger rim in combination with lower sidewall tire.

If you car has ample power- a twin charged MCS with LSD- then larger wheels isn't as much of a penalty and you can make better use of the wider rims and tires. Weight isn't an issue. For those with a mid modded MCS a light weight 17" or 18" is at about the limit for track use.

For autocross we use 15x7 and 225/45-15 or even 205/50-15 while with 16x7 we can use 215/40-16. These all work reasonably well.

 

minihune, that is some great info and good job on obtaining and sharing it.

 

larger diameter wheels=heavier wheels is a bad equation, Onasled had explained very well, it all depends on the wheel choice and tire choice.

It is an ongoing mistake caused by the lack of knowledge led a lot of NAMers against larger diameter wheels. to sum it up larger diameter rims do not mean loss of power in all cases.

 

I have 18" RacingHart wheels on my MCS and it's awesome.

I had the heavy 17" S-Lite before and my 18" are lighter. (Forged RacingHart)

On the road, it handles better. The car's response is quicker and more nervous. Less rubber ---> more feeling and quicker response from the steering wheel...

And it fits so well !!

Just as a "scientific" proof about the look :

 

...................... The 225/45-15 is 23.0", the 205/50-15 is 23.1", the 215/40-16 is 22.8"

The smaller diameter tires improve acceleration, braking and handling. Of course given equal tires, etc etc etc.

 

The valid response is that you can fit bigger brake rotors and calipers if you give yourself more room with larger rims.