[rotorwrench]

Flat land cars with manual transmissions all had the 3.91:1 (some had 3.92:1) whether they were standard or overdrive. The 4.27 was specialized for mountainous terrain driving. The 4.55:1 was optional for trailer tugging or the like. When the Merc-O-Matic was introduced, the 3.31:1 and 3.54:1 rear gears were used depending on the customers application but taller gears could also be used but not recommended due to the nature of those first generation Borg Warner automatics.