Roscoe,
we all should do this, even if the dynos are said to vary +/- 15%.
So it's okay for back to back testing and better than nothing.
If the dyno reads 300 hp, how would you know if this was at the upper level of the tolerance or lower or the real value?
Or if your engine had 300 hp that could show either 345 hp or 255 hp on the dyno.
So your reading maybe 345 hp one day. But as you do not know if this was the upper value (it could be the lower reading also: minus 15% of the real value).
Than your engine may produce 85% = 345 hp: 100% = 405.8 hp.
That is quite in interesting mathematical game, but you may get the picture ...
The drivetrain loss is more with automatic, or in any other gear than 4th (direct) of a manual. The sum is more like 12%, but I thought the dyno calculates this during the "coast down"?
Now, you dyno people: yell at me!
I am at least six hours of flight time away from you
And I am no fan of this "put your rear wheels in the roll and dyno" to find accurate numbers.
Dominik