Gears

---

A 9" Ford has a larger pinion vertical offset (distance measured in inches where the pinion meets the ring gear vertically in relationship to the ring gears centerline.) Examples: Ford 9” 2.25" is a large offset, Dana 60 1.125" is a small offset. The larger offset creates more pinion to ring gear tooth overlap.  

What this means is that if a Ford 9" and a Dana 60 both had the same ring and pinion tooth count, like 5.38s, the Ford 9" has about 3 teeth in contact with each other whereas the Dana 60 has less than 2. The teeth are the same size but there are more overlapping with the Ford 9". But in the case of a 4.88 ratio there would be so much overlap that Ford went to a 4.86 that has 1 less tooth on the pinion, meaning the teeth have to be thicker to cover the same area, but still has equal tooth overlap to the Dana 60 only the teeth on the Ford 9" are .040" thicker at the tip and .090" thicker in the center of the teeth where the load is applied. 

That's why Ford 9" gears are sometimes off a little in gear ratios like 4.57 compared to a 4.56 of others. All gear manufacturers choose the tooth count that gives them the best overall strength combination between tooth overlap and tooth thickness.