You can't use the rears on the front on XP1000 or XPT. The 8"(6.5+1.5) will rub. But a miss matched set of fronts with rears in back same tire is still OK. The slightly smaller diameter rear (if it even is) is not enough to cause problems with the front differential. I have two cars with fronts all around 30" Mongrels. Love it. If you do this the rear axle is 1/2" shorter on each side than the front. Don't worry about it. It is designed that way to help turning and tracking. The 8" rear rim center is closer to the midline of the car to give this effect too even though the edge of the front and rear tires line up. Don't put in a spacer. Too much stress on rear axle.