Still need to verify the operation of the temp sending units, but I thought I'd post a picture of the temp gauge I took after 30 minutes of freeway driving on a sunny morning.

Does it seem 'right' to you. Seems cold to me, but, I still need to check the O2 sensor to see if that's the reason it's running rich and the temperature is something else entirely.

image: https://skitch.com/apsoto/eu861/img-3993

 

Just as a follow up for future folks.

I replaced my thermostat with an OEM unit ($25) and although the temp seemed to get a bit 'warmer' it still didn't solve the problem. Poor gas mileage still persisted (8-10 MPG).

As suggested, I picked up a replacement O2 sensor on and my MPG jumped to over 13 on a tank that was 3/4 full before the replacement. I expect it to go higher once the whole tank full with the new O2 sensor.

Some key symptoms for future reference: poor gas mileage, no trouble codes thrown. It seems like O2 sensors do go bad over time like anything else, so it's relatively cheap to replace if it's got 100K+ miles on it.

Thanks everyone for the help.

 

Just another followup for folks. I've got a 3.5" lift and run 31" tires and I'm getting 21 MPG on the freeway. Looks like replacing the O2 sensor really helped!