Tire size and Gross weight do both effect which gear. MEasure total height of the mounted rim, and subtract out for the difference putting a load on the tires would have on the measured, unloaded height. An alternate meathod involves measuring the distance travelled in one revolution of the rear rim mounted on the truck and doing the math, if you hacve a tone wheel, (not for a speedo gear). It's more accurate, but a real pain to do.

GBE

 

Thanks for the input. I guess I'd need the 18 tooth then & the 17 tooth if and when I run 35's.