The 12 bolt design is a bit more efficient than the 9". Guys who use that are typically lightweight, not tons of power and willing to do whatever it takes for the last bit of performance. Think Comp.
Anybody ever seen a definitive/scientific test of rear end drag? Never heard of one. There was a magazine that did some tests, but as unscientific as it comes.
In fact, that took so much effort I am astounded that they did it when it all could have been done on a bench.
As such, this power robbing 9 inch story carries on and on, and will never end.
I find it amusing.
There have been tests & comparisons, but I don't have a link. The differences aren't a lot, but they are there. Most has to do with the pinion position relative to the ring gear. The closer to center the pinion is, the less contact surface the teeth have, thus less friction. Ford 9" also has an extra pinion bearing, I believe. The order I remember is : 12 bolt, Dana 60, 8 3/4, Ford 9". I don't recall any other rears, but the 10 bolt would probably be pretty efficient, if you didn't have a lot of torque/HP/weight.
I believe Jim Kinnet had a 12 bolt in his SS/AA cuda and a fast Stock class racer had a 8 1/4" rear in his car till durability issues and lack of ratios encouraged him to change to a Dana 60. This is also partially why many haven't seen a performance loss going from a 8 3/4 to a Dana 60. It may be bigger & heavier, similarly optioned, but the Dana is more efficient, especially if you take the weight difference out from elsewhere in the car.
