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The national highway speed used to be 55 MPH, and they were circled to notify you that was the max highway speed when these trucks were new. Worthless now days though. I've seen many vehicles made from the mid 70's to the late 80's with 55 circled.
Yes, back in the '70s they made the national speed limit 55 mph. If I remember correctly, it was under Carter that they changed speedometers to only go up to 85 mph. Before that, most passenger cars had a 120 mph speedometer.