After beating my head against more then one wall, looking for tires to put on a 18" Factory rim, I found this.....And found it helpful

http://members.aol.com/agspeed/tiresize.htm

Hope it helps someone else like it did me...Only issue may be the largest size it works with is a 305

Heres a few other Calculators I found that are helpful:
http://www.4lo.com/4LoCalc.htm fordtough_k_w Scroll to the bottom

This one may help with Standard Measure Units:
http://www.csgnetwork.com/lttrucktireinfocalc.html

 

do they got one for american size tires?

 

this works pretty good- http://www.miata.net/garage/tirecalc.html