I'm looking at an after market rear tire in a 110/80-18 size. Will this tire be a little taller and narrower than the OEM tire?

 

I'd guess same width and not as tall. Tires with the same numbers on the sidewall will not necessarily be the same actual size.

 

They will be darn close. Most manufacturers have sites where they have the actual aired up size listed. Here are a couple that I hope are still good. You have to do a minor amount of digging from there.

Shinko - Welcome to the Korea official homepage of Shinko Tires

IRC - IRC TIRE world wide top page

Bridgestone - Motorcycle-Karttires I recommend the Spitfire. At the lower right there is a spot for details or something, that is where the specs are. The Spitfire S15 is a dual compound tire found frequently on vintage bikes and builds. We sold a ton of them when I worked the bike shop in the 80s. Still strong seller in general.

Dunlop - Dunlop Tire Catalog I forget where it's at on the Dunlop site.

Duro - http://www.durotire.com/products.php has a relatively decent tire. A harder rider might want something a bit more sticky.

As for the size of the tires, the general rule is the width may vary up to 5mm wider or narrower, that's nearly 1/2 inch, so again look on the sites. The profile may vary a few millimeters but is a % of the width, thus the 90% of a 110mm would put the profile at 99mm with some allowance, but probably not a plus/minus 5. It should be tighter than that.