12 mpg is this normal for a 02 5.3????

I just bought a 02 Sierra Z71 w/71000 miles the first 2 tanks Ive averaged 12mpg. This seems pretty bad. Ive got a 01 suburban that has always gotten 17mpg. Can you have a code w/ no service engine light? Thought maybe a O2 sensor might be bad. Thanks for the help.

I have the same problem but my truck is an 01 with 50k on it and it consistently gets 12 mpg. Im changing the spark plugs now seeing if that helps it.

Hi,
I've started wondering out loud if the piston slap (knock) these engines get could be causing the knock sensor to activate and retard the timing. That would cause worse mileage.
Bob B