why would my gas mileage drop
 

I have been getting a consistant 28-32 mpg over the past 6 months and now I am lucky to get 25 usually 23-24. I keep my tires aired up, proper oil changes and other things along those lines. Why would I drop so badly? Same commute, same driving style if not better than normal. NO check engine lights.

10% ethanol or 15% ethanol.. How many miles on the car? Tune up time? Winter weather, longer idle to warm up the car? Gas station pumps not calibrated short changed you in gallons pumped.

up to 10% ethanol almost always use the same gas station since the gas light comes on at the same place every week. I just rolled 72k on an 07. I dont let it idle long before getting in it to drive, no more than I do when it is warm. I bought it used so I dont have any idea how the maintence log was before I got it. Its got me stumped. I probably need to replace the struts and shocks on it but I cant see that changing the mileage that much.

What brand of gas (BP, Exxon, Shell, etc.)? Ethanol content varies in each shipment of gas. Maybe your gas station has already implemented the 15% Ethanol and not changed their product signage.

usually shell or exxon depending on what side of the road the light comes on at.

I'd check the usual culprits like tire pressures, the air filter, whether the gasoline has been reformulated for winter, and for a curve ball even the alignment.

I would think it would be a gradual decrease in mileage, but what about carbon build up?

Probably not CCK, these engines run so close to the ideal 14.7 to 1 air fuel ratio I doubt they can really carbon themselves up. Back in the good old days engines did carbon themselves up since you invariably ran rich at certain engine speeds, like puttering around town. We still say we're "going to blow the carbon out", but in reality all we're probably doing is blasting some deposits off the plugs.

As another thought, TXhhr83 mentioned in another post about possibly trading his HHR in. Sometimes the act of falling out of love with a vehicle can cause your mileage to drop as you subconsciously "hammer" it for the lack of a better analogy.

Possible muffler baffle came loose and is restricting exhaust flow.