If you are using a 4.10 or lower (higher numerically) then the D60 is more efficient. The lower you go, the more efficient the D60 is over the 9.
Changing gears is pretty silly unless you are in Comp (maybe) or Pro Stock. I went through that gear change phase in the mid 1980’s and the only thing that happened was it changed the crossing RPM. Never made a tick of difference in ET or MPH.
So I stuck with the 5.57’s even though the math said 5.86 is what the car should have wanted.
