Congratulations on your new purchase. I did not find the fuel economy to be any better between my E53 4.4i and my E70 4.8i. Since your new X5 is a six cylinder I suspect that's why you're experiencing better fuel economy.

E53 is a great vehicle. While I currently own an E70 the E53 will always hold a special place in my heart. I especially love the look of the E53 4.8is