The v-8 has more power, thus it doesn't hafto work as hard or turn as many rpms to create the same power as the v-6 would. In a thread around here, people were saying the mileage is pretty equal. Search for it, and theres even a thread where people post there engine, and what gas mileage they get.
Edit.. check this link out.
http://www.automotiveforums.com/vbul...d.php?t=452684