Sticking a piece of cardboard over a portion of the radiator was a common sight during the winter when I was growing up in rural Ohio. I didn't think our winters were that cold, but maybe late 70s to early 80s vehicles were more susceptible to running cold.

I had a car that developed a stuck-open thermostat and did the cardboard trick to get by until I could replace the faulty part.

I've had that happen, too, on a [more] regular car. I drove a Mustang 5.0 from Oklahoma to Oregon, and as I went through eastern Colorado the coolant temperature steadily dropped until it was resting at the bottom of the gauge. I don't recall whether the gas mileage suffered noticeably or not during that phase of the drive.