For research sake a trip to southern Texas should get us 45mpg. Might be worth moving south...:propeller:

Yes I notice the extreme cold kills the MPG on this car. I normally get 29/30 on the highway, light hills 75mph, but when it is -9 like it is today I am lucky to get 24/25.

Also this car seems very sensitive to 10% ethanol blends. In MI we have 10% in our gas, if I travel to a state without that the MPG goes up. It is all relative.

Pretty well documented more ETOH=lower MPG.
Many localities change the percentage by season, that's what 843de keeps trying to tell people.

I still haven't figured out how the temp plays a roll, did it again today. One tank of gas all highway 120 miles at <20F got 25 MPG, return trip >32F got close to 30 MPG.
Maybe transmission fluid gets stiff? I did notice some odd shifting behavior today.

Could be the cold air is messing with our airflow sensor or the O2???

One would think cold air = denser oxygen rates right?!

The colder temps means the engine takes longer to heat up, remote starts cause 0 mpg and the heater is running to try and warm up the interior of the vehicle.

colder air to me should seem as to better performance, no car likes hot a$$ air being put through it confuses me a bit but who knows, shouldnt the colder weather be keeping everything like tranny oil, antifreeze and such cool to a good performing level?

Could be turning tranny fluid into molasses too. Who knows, just observations.

Another long highway trip today Fredericksburg,VA to Winston-Salem,NC. About 300 miles each way. Trip down starting at 3:30 AM ambient <40F ; 28.7 MPG. Trip back 8:00 AM ambient >60 ; 32.7 MPG!!!!! That is spreadsheet calculation, the DIC was lower.

This trip is all Interstate Highway except for 7 miles, did not get much traffic in the AM rush in W-S, only about 5 minutes slow traffic. AVG speed 65 MPH cruise control mostly set at 73 MPH.

Colder air is denser than warm air, so it is easier for the HHR to punch a hole thru warm air.

My HHR is the same as others, much better mileage when the temps rise. Don't forget added drag from stiffer grease in the wheel bearings too. Alternator bearings also.

As Snoopy alluded to earlier in this thread: the DIC MPG reading is always "dead nuts" correct. The amount of fuel injected into a cylinder is governed by pulse duration - the amount of time that the poppet in the injector is opened to allow fuel to pass the nozzle. This is always a precisely known quantity, because the dimensions of the passage in the injector is fixed, and the delivery fuel pressure is tightly regulated, so the flow rate is constant. Only the amount of time that the injector is open is variable. There is no question of how much fuel is burned when the engine is running.
If the outside diameter of the tires on the vehicle is the same as when the vehicle left the factory, then the distance the vehicle has progressed is a known quantity. Let's not get into tread wear, which decreases the diameter - it is too negligible to even be considered.
The ECM adds up the amount of fuel burned (whether moving or standing still), and it totalizes the distance traveled. Distance traveled divided by total amount of fuel burned is miles per gallon, or kilometers per liter, or whatever. There are no estimates, presumptions, or corrections applied. It's how much fuel you burned (not put in the fuel tank) and how far the vehicle has progressed (forward or backward).

The amount of fuel injected into a cylinder via electronic fuel injection is determined by a complex algorithm developed using inputs from various sensors. The objective is always maximum efficiency within acceptable exhaust gas chemistry parameters. Greater fuel consumption in cold weather has very little to do with the operation of the engine if all sensor inputs are within specification. There are dozens of variables not connected with engine operation which make decreased MPG inevitable, which is a whole other topic.

The ambient temperature sensor is definitely an input to the ECM. If you park your HHR and the ambient temperature is, say, 30 degrees, and you start the engine up some time later and the ambient temperature is 40 degrees, your DIC reading will still read 30 degrees until you drive a certain distance, and then the ECM will update the DIC display. If you park your HHR and the ambient temp is 30 degrees, and then start it later and the ambient temperature is 20 degrees, the DIC will show the correct ambient temperature immediately upon starting. This is done to prevent the engine running in an open loop mode due to a failed sensor input - very unlikely since the ambient temperature sensor is a precision thermistor that cannot be wrong unless the circuit has no power.