Hi,

I purchased my X used last month and the stealership had to replace the rear tires in order for it to pass inspections but they did not replace the front ones as they must have just had enough tread to pass.
They threw a different brand of tire on the rears.
In the meantime I purchased a set of 20's for my X which are heavier and wider and I'm getting better gas mileage with the new wheels. Go figure.
I guess it really does make a difference if you're using two different brand/treads on your X. It could also be that the front ones were almost run down and the rears were brand new? I wouldn't think that it would make such a difference but the new wheels are proof.