Yesterday it finally got above 0° F, so I spent some time outside and cracked the hood. I found that the air intake box clips had come undone, and after replacing them, the trip to pick up my wife from work improved from 56mpg to 82mpg. Part of this can be accounted for with intake air temperatures - it wasn't drawing warm air from my duct behind the catalytic converter with the air box ajar - but how much?
Also decided to install some shields to prevent salt and sand from being thrown up onto the back of the engine. There are openings in the wheel wells, presumably because that's where air is supposed to exit the engine bay, but parts of my engine bay were basically getting sandblasted. This is probably why the brake lines rust on most northern Insights.
My solution:
