Brake Horsepower can not be measured at the wheels. A lot of times a brake dyno destroys the engine (or damages it severely) in the process of measurement. A brake dyno is considered to be the most accurate. A wheel dyno doesn't factor in fluidic and other efficiency losses between the crankshaft and the wheel. I suppose it might be possible to build a brake dyno for a wheel, but nobody would be willing to destroy their engine and drivetrain to try it.

Calvin