I find it is much, much better
to talk about MPH per 1000 rpm
instead of differential gear ratios.

If you are turning 2000 rpm at 80 mph
your ratio is 40 MPH per 1000 rpm,
etc

Doing it this way combines tire size, overdrive ratios, and differential gear ratios.

In highway driving there is considerable fuel economy improvement up to a ratio of 40 MPH per 1000 rpm,
slower improvement above 40.