My dad has the same engine in his truck and it gets around 20-22 mpg hwy .. mine is way down to 10 to 11mpg. I have lots of power, no check engine light, normal temps, no gas smell. Air filter is clean. Have had this problem for a while but with the price of gas now it is really starting to hurt the pocket book. Thanks for any help on this.

 

FIRST,I DOUBT YOUR DADS TRUCK GETS 21..maybe 18 if granny-driven.

it needs to be scanned..not just a code retrieval,but by someone who can look at the fuel trims and decipher if its rich or not.

 

very likely its a simply vacume leak, mine had a dry rotted boot off the PCV valve and was idling rough and burning way to much fuel. it could also be an exhaust leak and its making the o2 sensors get a bad read

 

has it sat for a while or something? b/c my 04 sierra temp. lost bout 2 mpg when it sat for bout 3 months. but i started driving it again and it went back to normal.