[halB]

Here are some useful formulas for determining theoretical tire diameters and percent difference between front and rear in a staggered setup. I hope that will help Xi owners to choose the proper setup that won't damage their transfer case clutch pack:

Give two tire sizes X1/Y1-Z1 and X2/Y2-Z2

Diameter for tire 1 is: D1 = X1*Y1/50 + Z1*25.4 (in mm)
Diameter for tire 2 is: D2 = X2*Y2/50 + Z2*25.4 (in mm)

% Difference between D1 and D2 is: (D2 - D1)/ D1 * 100 (in %)

For example:

Tire 1 for the front is 225/40-18
Tire 2 for the rear is 255/35-18

Diameter of front tire is: D1 = 225*40/50 + 18*25.4 = 180 + 457.2 = 637.2 mm
Diameter of front tire is: D2 = 255*35/50 + 18*25.4 = 178.5 + 457.2 = 635.7 mm

% difference between front and rear diameter (D1 and D2) is:
(635.7 - 637.2)/637.2 * 100 = -1.5/637.2 * 100 = -0.24%

Which means the rear tire is 0.24% smaller than the front tire.

For Xi the common believe is that the system can tolerate up to +/- 1% difference between front and rear without causing damage due to excessive transfer case clutch pack wear.

Please note that not every brand and model of tires are created the same. Even at exactly the same size spec, some brands and models may be slightly larger than some other brands and models. If you are using the same brand and model front and rear as you should, the difference in ratio between front and rear should be insignificant since they both err in the same direction, therefore the size variation will largely cancel each other out in the division, at least in the first order of magitude, unless they are of very significant diameter differences which should not be the case for an Xi.