Use the fitment chart....it tells you what will work.

I think you will find the tires that are listed in "inches" have a significantly different (vintage) design than the modern metric sizes.  They tend to fit on narrower rims and be more like an old balloon bicycle tire with a tall sidewall.

Hey Rat, what type of rear end are you planning to stuff that thing in. I'd assume that you've got a hardtail, a heavily modified swing arm or a completely different frame. Keep in mind that your side to side clearance will also be a factor to contend with. Depending on its length and size a fender will be wider than the tire. If you try and run the widest tire you can fit you will likely need to notch the fender for chain clearance. Also as far as "wide" 18" tires that will properly fit on that narrow rim your options will be limited. The Coker Firestone replicas are the obvious choice. Also don't rely on the manufacturers specs for the diameter or width they claim their tires spec out at. My Coker and 4.50*18 measures just barely 4" in diameter and not quite 27" OD. It is mounted to a "factory spec" 18x2.15. The Firestone classics come out a little closer to their spec width but it's still really more the sidewalk profile that give those tires their big Balloon look.

FDM, thanks for the heads up on the Coker's. That is definitely the "look" I was going for.

this is going in a custom hardtail, using an S40 (LS650) as the donor.

I was looking to get the widest possible rear tire without sacrificing the aesthetics of the look I'm shooting for.

The front tire is 100/90-19 on a 2.15-19 and was hoping to find something a bit wider for the rear tire. This will also "help" with getting some ground clearance under me, I hate scraping, this isn't a lowrider  

The old school (late 70's, early 80's) GS1000 and GS1100 front fenders should give me the clearance on the rear that I need, with a little modification.

NEVER ARMOR ALL THE TREAD OF A M/C TIRE!!!!! NOT EVEN THE ''CHICKEN STRIP''.
It soaks into the tire and preserves it, great for bead and sidewall, BUT LIKE GREASE TO THE TREAD! It will make the tire, font or back, slide out like you hit oil or ice.

I'm not knocking on you, just can't be too strong with the warning about AA on the tread.