fuel economy

What is your question?

I recently purhaced a GMC Serria 1500, 6.2L 403hp pickup new from a dealer. The first week using the gasoline the dealer had in it I got almost 15mpg. The subsequent 4 or 5 tanks of gasoline it gave me 14mpg acording to the average mpg gage on the vehicle computer. I made it a point to drive it conservatively. I am very happy with the vehicle performance It's a hotrod so to speak but I thought this was curious and wondered if they were possibly using regular unleaded visus 10-15% ethanal/regular unleaded fuel causing this difference.