I ran a 245/40/17" wheel on my Fox Body with a 2" drop and had no clearance issues.
Look up that tire on Tire Rack and see specs for overall diameter. This will give you a good idea of max hight, but you can go a bit bigger, I'm sure.
As for not messing up speedo, you will need to look up the OEM tire size and see overall diameter and keep new tire the same size. Fixings speedo is not a big deal as I needed to with the tires I mentioned above. They were 1" smaller diameter than stock and was 5 MPH off at 70 MPH. There is a small gear you change on speedo cable going into side of trans. This was a 15 minute fix and was just fine.
Jazzer
