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.

Should get at least 17mpg same as your burb.
I would start and change out the air filter, fuel filter and spark plugs...then take it in to a dealer and ensure everything else is up to snuff..ie: timing, clean injectors.. etc.
Mines a 05 Z71 with 4.10 gears/5.3L auto and it gets a consistent 23mpg (cdn) highway at 110kph.

My 99 5.3 Silverado gets 12 to 14 MPG around town, always has.

Does seem kinda low i have the exact same truck and i am getting about 17 mpg as long as i keep my foot out of it. i agree with tykrz start with a good tune up and i assume you are driving without flooring it at each stop sign.

lowest i ever got was around 15mpg in my 02 silverado 4.8... Then i kept getting 17 mpg after awhile i change my driving happens. When i start from a redlight i try to keep the RPM under 2k and never try to gun it unless i have to around slower traffic to get on the interstate. Seems to work because lately i been getting around 20mpg.

What size tires you have?

thats about what i get also. i have the 4:10 gear though. i will get 14 in the summer. less idling and stuff (michigan winters are cold). i'm not sure if the gas blends change with the seasons around here anymore, maybe that has something to do with it if they do.

i asked my mechanic at my last oil change to check the spark plugs and he said they are fine. i have 49,000 on it and he said he wouldn't worry about them for another year.

I've got 285/75 tires. I had it hooked up to a scanner today but he said everything looked fine. I did forget to consider that I was actually covering more miles than the odometer indicated because of my tires. I dont know how much difference that makes but it will help my average some. Im not an aggressive driver at all. I just ease around.

i do 75 % highway and 25 % city and i get about 17.5 mpg with a 5.3 not sure about my gears and all but...

A suburban is more aerodynamic than a Silverado, and unloaded, will (should) get better mileage. My '02 was getting ~12 MPG before I got my Nelson Tune, and now I get ~14.5