The calibrator will do nothing about the torque converter lock up or lack thereof. You either need to run stock tires or regear. You also have the option of regearing the t-case vs. the diffs. Georg and Valley Hybrids can help you with that. I ran 4.88's and 35's for years. The engine revs higher on the freeway but the tradeoff was worth it for me. I pulls the hills in SoCal much better being undergeared. However, if you lived in say Utah and did 80+, well that screaming engine gets annoying so 4.56's might be favored. It's really up to you to decide the balance of tradeoff's.
As for the math. It has to do with the circumference of the tire as defined by the distance from the axle to the ground. A larger diameter goes farther with each turn of the wheel. Your speedo is simply measuring turns of the driveshaft which is calibrated to the stock gear ratio and tire size. When you increase the tire diameter that calibration is now inaccurate as you are going farther with each turn of the wheel than with stock tires. The actual equation is S=r * theta. S is the arclength, r is the radius (distance from axle center line to ground) and theta is the angle of rotation in radians. However, if you were to find the ratio between larger and smaller tires, theta cancels and you are left with the radius of the new tire over the radius of the old smaller tire so the difference is simply new radius / old radius.
You can also just look it as a difference of circumferences. which is 2*pi*r or pi*d (diameter). In this case pi cancels and you are left with the ratio of diameters. This will give you the same answer as above but let you work with diameter instead of radius. I believe the truck has 31" tires originally. 35/31 = 1.129. If you multiple 8MPG * 1.129 you get 9MPG. That's because you went more than 8 miles / gallon, you went (8* 1.129) miles / gallon or 9 miles / gallon.
Frank
