Vector, if horsepower at the drive wheels remains constant, the time it takes to cover a distance will increase as the weight of the vehicle increases. It's true that you could calculate a total amount of horsepower performed during a run if the only work you were concerned with was moving the weight of the vehicle. Say you have two track surfaces. One is absolutely slip proof, the other is an oily surface. You run the same exact car on each track. It may take you 10 seconds to cover the distance on the first track, and it may take you 20 seconds to cover the distance on the second track. If you were simply concerned with the power actually used to perform the task of moving the weight of the vehicle from the start to the end of the track, you used 1/2 the amount horsepower on the oily track.
In reality, though, the engine was still capable of putting out the same amount of power on each track. Power was just wasted spinning the weight of the wheels and generating heat on the oily track.
It may be that the Mustang Dyno is geared toward diagnosing drivetrain vibrations or other issues that you may have a need to crawl around the vehicle while it's running at highway speed under load. If that's the case, that's fine, but if technicians are trying to convince people that it gives you a better way to tune your car for horsepower, it's hogwash.
The horsepower we are most interested in is what is available at the rear wheels under a standard condition. This is what a DynoJet will give us.
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351W 89 Mustang GT Convertible