My /6 is a gas pig when it's cold outside. Why?

Hi All. So I've got this issue with my 73 Dart. It's running great and when temps are above 40 deg it returns very reasonable gas mileage, even with multiple cold starts between fill ups. I've only had one reading below 20mpg since April, (19.8mpg) and my average has been 22 for combined city/highway/back roads. But when it gets near freezing, this thing is an absolute pig for the first 5 miles or so. So much so that I get out and check for leaks because I can't believe that the fuel gauge needle has moved so much in so few miles. I don't notice any black smoke in the rearview mirror while this is happening. I've experimented with timing, choke settings etc, to no avail. Do I need to let this thing warm up for a few minutes before I start driving? Timing is currently set at 2 1/2 btdc and it's fully off the choke generally within 3 miles. Any thoughts or is this just the way they were back in the day? I would expect some decrease in efficiency with cold weather but 30% or more seams a little nuts. Thanks

AL