14-15 is about max opn an older p/u..

 

I have the 5.7 two wheel drive. My brother told me to expect 20.9 to 22MPG on trips (I drive between Texas and Oregon). Surprisingly mine has seen near 20MPG. When I first got it the 4L60E needed rebuilt. While it was trailered and towed with my Dodge Cummins the mileage was about 20 (ya, I know... a Dodge!! Something I have hated most of my life!). I love the leg room in the Chevy and if it didn't sound so good when the throttle gets pushed I would do much better.
I have been practicing restraint and try hard to let the converter stay locked in with the RPMs around 1200 to 1300 but it is all I can do to just keep my foot out of it for short strips of highway. Maybe when I grow up I will get better at this restraint thing. I'm still just a 55 year old kid in the Chevy.
Abe