One possibility is an auxiliary tranny.
http://www.6066gmcguy.org/spicer-5831-b.htm Fumblin 46, the answer is yes. It is a function of the diameter. The tire you mention has a 32" diameter. If you are now running 6.50X16, they have a diameter of 29". With that you get a difference of 3". So your speedo will be reading about 10% slow, from 3/29= about .10.
As an example, my 52 3/4 ton I went from 4.57 gears to 4.10. I also went from 7.00X15 to 7.50X16 tires. My speedometer reads slow by 18%.
