This is how most enthusiasts look at it.

For example, a "Low" first gear provides a lot of grunt but tops out at maybe 3, 5MPH in a big rig.

A high gear ratio is numerically low, but it allows for a higher top speed. I think this is where the high versus low comes from (your wheel speed is high or low).

Think about 4x4s with "4 LOW" where your gearing is maybe 4:1, versus 1:1 of "4 HIGH"

Anyway, I would suspect a 300SD is geared the way it is for a few reasons, one of which is potentially aero drag and horsepower. Would a taller gear allow you to go faster, or would the drag of the brick, combined with its lower horsepower, result in insignificant changes in top speed with a stiff penalty to acceleration and on-ramp performance?