I was wondering if 18 in rims with 245/40/18 would be the same size as stock 17 in rims with 215/50/17, since 235/45/18 is hard to find?

 

Sounds big. I don't have my wheel calculator site handy, or I would look it up.
I am running 235/40/18's, and they are close to stock 17's. 245 would just make them taller. Plus, that is WIDE.
I think someone posted the tire size calculator on here... some miata site, if you want to search for it.

 

215/50/17 = 25.46
245/40/18 = 25.72

A little on the big side. Depending on your wheel offsets, it might work. You will need a rim that's 8-9.5" wide.