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

On the octane.....Not really.... it ought to run on anything. Is it doing this at lower and medium speeds, and does it not do this if you are at 55 mph and floor it, for example? Pinging will be more common at lower engine speeds, if you (or the trans if it is an auto) do not downshift, and the engine starts to lug. And, it will do this more easily with advanced timing and a lean carb. The Dart Lite's and Feather Duster's in '76 were more prone to pinging with their extra lean carb.

1) Your 26 mpg on that run kinda makes me think your carb is on the lean side, especially with just a standard 3 speed manual or auto trans. I would check the float level when running (look up 'wet float; setting) if this is a Holley 1920, or at least do a regular float setting for either brand of carb. Too low of a float setting will make it lean.

2) Also, what trans do you have? If a manual, you can downshift sooner and avoid this. Or if an auto trans, the downshift linkage to the trans may be misadjusted, delaying downshifts and causing the engine to lug and thus ping.

3) Very slight vacuum leak will lean the mixtures but this would not tend to happen with open throttle settings.

4) The engine's timing marks may be in error and causing you to have more advance than you think. This would give you better economy but more chance to ping.

5) The power valve (economizer valve on the 1920) may not be working and so you end up with a lean mixture when you run at open throttle. That would help gas mileage too, but cause lean running and pinging under hard throttle. The car would also be doggy at hard throttle.