Like it says I have a 2002 Suburban 5.3 Z71, it s on 33" tires properly inflated. Has 200k on the clock but no codes stored, no miss, uses very little oil.
I replaced the plugs and plug wires, all filters, syn oil and run premium fuel.

I am getting about 10mpg in the city. I checked the speedometer/odometer against a GPS and it is reading correctly.
I'm not driving like a maniac, but nothing I do seems to get better MPG. Goes up to about 14 on the highway.

Everything I read says I should be getting 12 to 14 in the city...

Any ideas?

 

33's will kill mileage some. I lost mpg in my '11 5.3 on stock tires. It dropped from 18-19 to 15 highway. MAP sensor was bad. No codes were being set though. It was off enough to drop my mileage but not quite bad enough to set a code and engine light.
It took me a while to find it. My high-end scan tool has a MAP sensor test program and I happened to be running the test one day and saw it go pass, fail, pass, fail. It was not far enough out of range to set a code.
I'm cheap and don't like to guess at spending money on sensors but glad I saw it screw up on the scan tool because it ran great and would not have believed it was bad until I saw something off.
I'm back to 18-19 on the highway when driving under 65.

 

great! I’ll scan it and check the MAP. I have full scan capability in my shop, I’m just not familiar with domestics at all. Thank you.

at $21 I’m going to test it and replace it regardless, seems good preventative maintenance.

 

Not the may sensor. I mapped the old one and the new one and all the readings were identical.
Baro is 29.8 and MAP shows the same not running, running MAP shows 11, seems to me it should be closer to 9 but it's not much difference.
Checked for manifold leaks by spraying the gaskets while watching MAP and no change.
FAM was 14.0:1
The MAF looked OK but I don't know how to get Actual v Requested values on the Solus Edge. I'll have to call Snap On tomorrow.

 

My Autoenginuity scan tool has MAP sensor tests. I also saw my dash MPG readings not going to the high 90's in real time mode when coasting downhill with foot off the gas which got me looking further. Also, my emissions monitors would not run the EVAP tests so I could pass state inspection. All went back to normal after changing my MAP sensor.
MAF, MAP and the banks 1 and 2 front O2 sensors are the ones that can mostly affect mileage.

 

new MAP. MAF looks good and throws a code if I unplug it, O2 sensors look perfect on the scope.

 

If sensors are good, it could be a partially plugged cat or exhaust system since mileage is high.
4x4? If so check to make sure front axle is unlocking. By that I mean left and right wheels are freewheeling in 2wd.That can pull down the mpg's if axle stays locked together.

 

Thought about that but there is no hesitation at all, snap throttle is good. I guess I can pull an 02 sensor and do a backpressure test, but typically a clocked cat will pull 02 sensor or cat code and the numbers all look good.
If the front wheels were locked I would feel it cornering and it just snowed here, I tested both the 2 and 4wd.

I replaced the MAF today so we'll see if it makes a difference...