whats the fatest tire you can put on a 91 z28 and what size rim
and what the offset is

 

gee i guess nobody knows

 

or everyone knows and would rather have you do a search, becasue someone asks questions like this every other day.
but I will be nice and tell you. Stock is 245/50/16 but you can run a 255/50/16 if you want to. the ofset is 0. +5 for the front on some. check out this link too for further size questions.
http://www.miata.net/garage/tirecalc.html



edit: or you could go look at the tires on you 91 Z28, assuming they have the stock wheels one them.

 

you can fit up to 275's thats what mw66nova is running, but his arent the Z28 rim though.

 

I am planning on buying 275-40R17's with 17x9 or 17x9.5 rims. Offset should be the usual one for our cars. I will be running 4th gen rear so I am buying adapters for the fronts and will be running with 4th gen offset.

Don't think you can get bigger than this without doing some hacking and massaging