Thanks guys. I am positive it is off; when it reads 70 I know for a fact I'm only doing 65.

I have my transfer case tore apart right now, and from what I can tell I have a 7 tooth drive gear. Using the above formula, I come up with 17.64. My tires are far from new, so an 18 tooth would be about perfect. Thanks a lot pdqford! You did a lot of thinking for me there. I didn't realize there were different drive gears also.