Well it bothers me cause its a stock truck. Stock. Only thing that has been done to it is things they claim to add mpgs. It seems I'm losing it. I've done a rear diff service about 6 months ago as well. Only thing I've never done is trans service. Scared to. Heard bad news about that. 175k miles and I got it at about 90k and I've never done one. And I'm not sure If the previous owner has. Keep hearing at that long it can break up the build up and mess the trans up.
What color is the trans fluid on the dipstick? What color was it at 90k? Not looking into it as soon as you got it and establishing some form of maintenance schedule was dead wrong.
About your mileage, how well does it idle? Is it perfectly smooth? Is there a miss/does the motor have even the slightest shake? No one has mentioned injectors yet, and if you haven't read or done any research
start here (link). Your cats could also be clogging up, there is a tool to measure exhaust backpressure, it installs in an upstream O2 sensor bung and there is a spec for acceptable exhaust back pressure.
My personal experience:
I had a 1999 Tahoe, got it in 2001 with ~20k on it, drove it until it had 170-180k and during the last SUMMER I owned it I put 30k on it over a couple months. It took a few years to hit the first 30k, at that point I did fluid and filter on the transmission, and then repeated it every ~30k miles faithfully. I put a throttle body spacer on it and an amsoil air filter, and I really don't know that they did much. I ran the spacer for 80k or so and took it off, didn't notice any difference. ALL of the tune up parts I used were Delco, the OE line, so platinum plugs, etc. At about 140k I noticed rough start and decreasing mileage, ended up putting the upgraded injector assembly in it, voila, better throttle response and all my lost economy was back. When I traded it in, at the 170-180k mark I was consistently getting 17.5/18/18.3 MPG with long stretches of highway.
