This is good advice, especially for your 3.0/auto since it is a known/common problem.

When the t-stat goes bad, it doesn't open when it should. This keeps the hot coolant from getting to the engine temp sensor. The engine thinks its still cold, so it runs a richer fuel mixture and also keeps the tranny from going into overdrive and lets it rev higher before down-shifting. It is designed to do this to help get the engine up to operating temperature more quickly to reduce smog emissions. This combo makes for very poor fuel economy when the t-stat is bad. The t-stat is cheap/simple place to start troubleshooting.

A co-worker of mine had these exact sypmtoms and the t-stat fixed it. He was grateful!