As a 2 year Prius owner (my wifes) and a recent Green Line buyer here is my 2 cents:

The cold does cut about 10+% off milage on the Prius, from 48 average to about 44. On my new Green Line I have gotten 33, 31 and 25.4 mpg on my first three tanks at about 60-75 degrees (TN/KY area). The last low milage one was in KY back roads and hills with lots of lugging the motor up hills and up and down speeds with corners. I have about 250 on this tank and just hit half, so I assume I'm back up to the 30's again in mostly highway/rural within 10% of posted speed limit with limited use of Cruise Control to flat areas. I've found that the cruise is the weak spot on MPG race if you come to a hill or long incline. It tends to down shift and race the motor and does not hold the miles per hour steady on a hill. I assume it is a "analog" system (for price) and will research more. It is the weakest link. On the Prius the cruise will hold like a rock on the speed you set (all digital system) and has the CVT.

Bottom line: You can have a mechanical reason for the poor milage, but more often than not it is driving style. There are a lot of tips on the prius chats, forums, etc..... not so many on the Green Line yet. Here are a few that may help:

On the Prius thay do what they call "pump and glide" to improve milage. They throttle up to speed and let off the gas until the Electric motor kicks in.....then feather the throttle up on Electric only to get max MPG. You can't do this on the Green Line!!! You can however thottle up.....let off the peddle, till the ECO light comes on and the RPM's on the tach drop, and then feather the gas......e.g. you get to 40 miles per hour....and then work your way up to 55 MPH at the lower RPM in the Higher gear when it downshifts. When you let off then throttle up, notice that the electric assist needle will peak up to assist the gas motor. If you just hold the peddle down you will not get the "assist" to work to the fullest capacity.

DO NOT USE CRUISE ON HILLS......you can control the throttle better than the cruise does on a hill. ON the flats....Use cruise as much as possible. It prevents you from un needed peddle modulation, thus, using less fuel.

Just my opinoin....PLEASE NO FLAMING!!!!

BOTTOM LINE: keep the RPM's down and figure out how to use the electric assist as much as possible and you will improve your milage. This car is designed for HIGHWAY miles per hour, not CITY like the "Advanced" hybrids. Short trips will kill your MPG.....as in most vehicles, even the prius if it does not get up to temp (more than a 2 mile trip) suffers the same problem.

Good luck!

Dundertaker