Maybe this has been discussed here already and I missed it. Anyway... I was at the accessory store today and picked up a Keystone flyer. While browsing through it, I noticed a gadget they have listed for recalibrating your speedo. So. . I looked it up on the web - it's rather pricy, but I guess someone who can afford to spend a couple grand to buy bigger rims and tires won't mind paying a little extra for a calibrator.
http://www.4x4cyberstore.com/website/tru_speed.html
Doc
http://www.4x4cyberstore.com/website/tru_speed.html
Doc