Related to last question.

[cblanche]

Since the thermostats were maintaing 160 degrees and no higher (discovered that one was bad), would that cause poor fuel economy? Or could my poor fuel economy be a result of 7 days of below zero weather?

It seems to be running rougher since this thermostat problem. My service engine light just lit up !
Could I possibly have another problem I overlooked?

[tom.mcinerney]

if the engine coolant is too cool, the engine won't run as efficiently. Some 'winterised' fuel can contain less energy, less "BTU"s.
If you notice uneven/rough running could be another issue...

[Uncle Wally]

cblanche - have your engine codes pulled. If the SES light went on, there should be a diagnostic code stored in your ECM. Not always does one get stored, but there should be one.

Waldo