I had my 2.2 Cobalt LS a few months back have the thermostat stuck open and the AC would not work as the computer bypassed it because it detected the cooling problem.

The work was done, car was fixed, plus an oil change.

The problem is this, I 'was' getting 33-34mpg for driving 60%fwy.

Now I get 29.4-29.8mpg for the same 60%fwy driving...

My driving habits have not changed, my tires are still filled to 30psi.
I use the same 87 octane and brand gas.
My car does not have any more stuff in it than usual.

What the ??? is up???

_______ALSO________

The wife's Sunfire which also has the 2.2 Ecotec, used to get about 30mpg on the dot with 60%fwy miles. All fwy miles landed up 37mpg.

She upgraded to the 2.25" exhaust and cat, plus and AEM CAI.
It went down to the 28mpg range.
A few years later, now she gets 26-27mpg.

I have used the injector cleaner every 3000 miles.
Always change the oil at 3000 miles.
Tires are at 30psi.
Fuel filter swapped every 15k.
The TB is sprayed clean every 9000 miles.
I have done the Seafoam in the fuel, oil and intake about 12k miles ago.
I of course drained the oil with the Seafoam and did not drive with it in.
New plugs at 95k miles, new axles and hubs both sides, cat new at around 95k miles, and the cone filter is cleaned every 12k miles.

What could be constantly dragging the MPG down on this Sunfire?
I know it is colder out, but in AZ it really isn't that cold.
It is getting 26-27mpg in the summer too.
The car is not driven hard at all, in fact, the wife drives very tame.

What is up???

 

I will reiterate:

The reason I am asking, is because magically my mileage went down after the thermostat was replaced and had an oil change.

All factors being equal except the above, now I have a consistant bad 29ish MPG instead of closer to 34.

I did reset my computer just recently thinking it would help, and it did not.
I want my 34mpg back!!!