Here's an interesting one for you. The 2001 Silverado Z71 I bought a month ago has pulled down 14.8 mpg on the first half tank which was daily commuting and not getting into overdrive or above 50 mph, then another week of daily commuting and a 130 mile highway trip(65-70 mph in overdrive) averaged out to 14.8 mpg as well!
My last tank with daily commuting came out to 14.8 mpg. I just changed the plugs, put in a 160 t-stat, a ported TB and took off the mud tires and put on some BFG AT's. Already it rides quieter and actually coasts when you take your foot off the gas. I think the worn mud tires were really hurting it big time. I'll see what this next tank does before doing some tuning, the LTFT's are running around 8 so I should be able to adjust the fuel curve and spark to get better cruise conditions. I need to see what the first set of changes did first though.