I have a '96 Plymouth Neon with a 2.0L four cylinder engine and an automatic transmission. I check my gas mileage regularly because I do a lot of highway driving.
I've noticed over the past two winters that I get noticeably lower mileage in the cold weather. Spring through Fall I get about 33mpg on the highway, but during the cold months it drops 10 mpg down to about 23mpg.
The people at the dealership say it's normal and won't even bother checking it out.
There could be something wrong, but not necessarily. Fuel mileage will normally decrease in colder ambient temperatures. Gasoline does not burn nearly as well in the cold so it requires the use of more fuel (a richer air/fuel mixture) to keep an engine running properly.
Another item to keep an eye on in the colder weather is the air pressure in the tires. The air pressure could have been correct in the summer, but the PSI will drop along with the temperature. Low air pressure in the tires will cause additional friction at the road surface, and believe it or not, this does affect fuel mileage.
That's the normal stuff that reduces fuel mileage, but your service department should have taken the time to eliminate any possible abnormal causes. The first item to check would be the operation of the thermostat. A thermostat stuck open or improperly calibrated can cause the engine to run below normal operating temperature. This will fool the engine's computer control system, and cause it to command the delivery of more fuel for a cold engine in warm-up mode.
This can also prevent the application of the lock-up torque converter which locks the engine to the transmission at highway speeds and has a major effect on miles per gallon.
Try another dealership if they refuse to check out the basics. A road test with a computer Scan Tool, and a quick run on the emissions analyzer is all it takes.