[Cabnchassis]

The national speed limit was supposed to be 55 basically during the years of the square body. It was changed in 74 and then changed back in the late 80s so that gearing probably drove just right for the time. That is if it was in one of the dozen or so states that actually cared to comply with the law. If you don't know your rear gear ratio it'll be on the paper in the glove box if you can still read it or it's even still there.