I read in Mopar Action magazine that the Mopar engineers chose their ratios based on several factors. One was in relation to the amount of times the pinion gear intersected the same teeth on the ring gear. Please forgive my vague description here, but the theory was that the fewer times the same teeth touched each other, the longer lasting the gearset was by spreading out the wear and the load. This explains why the Mopars came with some ratios different than GM or Ford. Numbers like 2.71, 2.76, 2.93, 2.94, 3.91 were rarely seen outside of the Mopar brand. the 3.23 and 3.55 were more common ratios though. GM used them as well as some GM specific #s like 3.07, 3.08, 3.42 and 3.73.