You need to use a BIGGER pinion gear when using the 1/8 scale diffs, not a smaller one:
bigger pinion and/or smaller spur = gearing up
smaller pinion and/or bigger spur = gearing down.
Thats why your speed is greatly reduced when using the lst diffs in the example above, becuase lst diffs have a lower ratio than the emaxx ones- you introduced a 2-folder gear reduction
I happen to use 43/13 diffs in my GorillaMaxx with 70/24 gearing on 5s lipo for ~40mph topspeed; redo your sums with an 18-19t pinion & the lst diffs and you will find it comes out about the same as the standard emaxx example
