Replacement tires wore out unevenly in 20k miles; Recommendations?

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Hi folks,

My 2003 X5 came with the stock 18" Michelin Energy which lasted about 50k miles and wore evenly. I replaced them with Kumho Ecsta STX because they had good rating for the money. Only 20k miles later, I have to replace the Kumhos: the inside 2" of both rear tires is completely worn, as if the X5 insteps at the rear. The front tires wore evenly. Since all four original tires wore evenly, I am thinking that the problem is with the replacement tires and not the X5 (allignment, etc.) -- not a good match for the X5? I am thinking about going back to the original Michelin. However, there are better rated tires out there for less money. Any recommendations? (I am not concerned with snow or ice, only rain). Thanks in advance.