When my pinion bearing began to howl at 46,000 miles,
I swapped in a $75 set of NOS Chrysler 3.21 diff gears in place of stock ratio 3.55 in a 1995 Ram 360 Magnum 2wd 518 auto where the truck weight was 5400 lbs.

Highway mileage at 70 mph
over the exact same I40 trip from
Durham to Wilmington NC and back
went up by
1 MPG,
and stayed that way for multiple trips.

One other thing I was not expecting:
2nd gear of a 518 with 3.21 ratio diff
is actually much better
than 2nd gear with 3.55
especially when towing up long grades.