how tall are 18's?

i wana set my truck up to lay on 18's probably a 45 sereise tire. how tall would that be?

http://www.miata.net/garage/tirecalc.html

it still depends on the tire width as well

yeah its all in the tire width... do you have discount tire in your area if so the have a metric converter that can help you get the exact width, side wall height and overall height.

Here ya go bro.
"http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp"

all depends on what size 18s...

and also, like they mentioned above - it all depends on tire width.

example:
215/40/18 is 215mm wide and the side wall is 40% of the 215mm width.
so
225/40/18 would be taller since 40% of 225 is greater than 40% of 215.

but if you are mocking up the ride with some scrap tires then get a tire that is the same height as the 18" tire you are wanting to run.

hope that was more help than confusion.