My opinion, the main reason for the two different ratios comes down to tire size. And it's so slight, Ford doesn't even force one over the other. If they did it would be 3.31 ratio with 18" & 3.55 ratio with 20". In tenth gear with 18" & 3.31 ratio at 65mph you're at 1423 rpm. 20" & 3.55 ratio at 65mph you're at 1428. 18" And 3.55 it's 1526 rpm. 20" & 3.31 it"s 1332 rpm.