I understand that you use a coast down test to determine flywheel power. Why, when wheel power is what makes you go?
The problem with measuring wheel power only, is that many factors can affect the wheel power output - tyre design, temperature and pressure, wheel alignments, gear box oil, viscosity temperature and level, wheel speed, final drive ratios etc.
As an example; take two very similar cars with similar modifications, both are tested on a 'wheel horse power only' dyno, Car A has 12 Bhp more at the wheels than Car B. The owner of Car B and/or the dyno operator begin looking for problems with the turbo chargers, ECU calibrations, CAM timing etc. If the test had been carried out on the Dastek Dyno, the coast down test would have revealed the transmission loss would have appeared excessively high, thus taking the focus away from the engine fault and pointing us in the direction if the drive line problem. The fault was in fact a partially seized brake caliper, which was converting the power into heat.