That seems odd to me.

I was impressed with my 2003's handling in the snow recently. Mine has 17inch All Seasons. Braking traction wasn't amazing but I wouldn't consider it bad.

Looking at the specs of a h2 its 2000lbs heavier and the stock tire size has a taller sidewall. If my math is correct the hummer has a 2.4inch (60mm) taller sidewall. I'm pretty sure increasing the sidewall while keeping the width the same results in more grip in loose terrain conditions. Of course it depends on the conditions, for example Ice is different than fluffy snow.

03 BMW : 235/65R17
05 H2 : 315/70R17

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2003 BMW X5 Manual