Gas mileage gong down hill

My '04 Envoy 5.3's mileage was 21 mpg a year ago. In the last few months it's been going down hill. I'm now getting 17-18. I did a tune-up, plugs and wires, 50k miles, that didn't help. Any suggestion?
Boy, with all the problems I see here, I should feel lucky that this is my only issue.:popcorn:

Do you have any codes, espaecially transmission codes?
It almost sounds like the transmission clutch control (TCC) is not working? If that be the case you will not shift into overdrive and will be in drive at best. In drive most vehicles have a gear ratio of 1:1. In overdrive it is 0.66:1. Do the math you will be using about 1/3 more gas. If I am correct you will have to drop the transmission to change the coil.

If you find a decent shop like I did for my old van it was $300 for the tranny in/out and $50 for a coil and filter change. The dealer will probably want closer to $1000.

There is also a few sensors that would throw the mileage out the window. Such as MAP, TPS, etc.

I guess you need to answer the code question before we could proceed?

try changing the o2 sensors, fuel filter, air filter and cleaning the throttle body. they're simple fixes that can save you a couple MPG's. i would do that before messing with the transmission.

I wouldn't touch a thing including any tune up stuff as suggested by others until the code question is answered.

In my experience, it is often the O2 sensor. It'll seriously screw with your mileage. I had a honda that went from 35 mpg to 20 mpg just because the O2 sensor was bad.