gas mileage has dropped...2004 Envoy XL 6 cyl

My 2004 Envoy has about 80,000 miles on it. It runs fine, but recently gas mileage has dropped severely. What should I check first, besides the obvious? I change the oil every 3,000, and it uses no oil. Air filter is clean and relatively new. I haven't checked plugs yet, but isn't this allegedly a 100,000 mile tune up car? I know the plugs, wires, etc. are a first place to check, but has anyone had a similar problem? Is there a particular thing that is known to happen to these things at this mileage?

Welcome to AF. Me too, cold weather has knocked off 2 mpg. here in the NE. Get it scanned and look at o2 sensor activity for starters. Use of awd or 4wd has the same effect also

Thanks for the reply...quick question....if I have someone hook up the scanner will it tell the 'history' of O2 sensor activity, or will it only indicate if there is a problem at that moment? Wouldn't O2 sensor give a Check Engine message?

The 02 could be " lazy" and not set a code.

I noticed a drop in fuel economy and found that the flange bolts between the exhaust manifold and the cat were loose, resulting in an exhaust leak. Once I tightened those back up, mileage returned. Open the hood and listen for a leak on the passenger side of the block.