To properly measure flywheel horsepower you need to remove the engine from the car and use an engine dyno. The calculations usually done to estimate flywheel horsepower from rear wheel horsepower, typically expressed as the reciprocal of a percentage of loss, can be misleading. (For one thing, the power consumed to spin the drivetrain at a given rpm doesn't change in relation to the power of the driving engine, so we know percentages are misleading.) Perhaps it's just me, but I've never been interested in flywheel horsepower, only power measured at the tires, because you don't drive on flywheels.