Surely the transmission drag is going to vary dependant on gear used as well as diff ratio on a rolling road. In this context whilst I agree to a racer the power to tyres is important, the only constant is the engine power at flywheel.
Having had my 3.7 Roadster and John V6's on a rolling road recently, the guys setting up our runs up chose to use 4th gear (of six) which gave a road speed on the rollers in the 120 - 125 mph range. A lower gear would likely have increased drag but since the session calculated this to give a flywheel figure, further corrected for temperature and atmospheric pressure, it would be irrelevant. As it was such a light car and abundance of torque caused issues getting grip to the rollers for both our cars.
We did consider afterwards if an extra body in the passenger seat would have improved matters, especially since my calculations gave a road speed of 132 mph at the max revs used in 4th.
