Here are a few, but the first one is probably what you are looking for.
http://www.4lo.com/
http://www.miata.net/garage/tirecalc.html
http://www.dsm.org/www.dsm.org.1998/tools/tiresize.htm
I know this may sound stupid, but I would think that with a 2" tire height divided by two actually would give you 1" in increased vehicle height. Thus you would have no clearance.
That would be based on static information, not including the dynamics of air temp and tire pressure.
Black Lingenfelter Lux