If you are wearing your tires out before 40K miles, then you need to rotate more often than 10K.
Why? Because you don't want to get too much wear difference between your fronts and your rears - and at 40K, you're on the verge of it being too much (2/32nds)
Why? Because you don't want to get too much wear difference between your fronts and your rears - and at 40K, you're on the verge of it being too much (2/32nds)