I person from a diff site said this.
"A dynojet dyno does not realistically load a vehicle for tuning. Dynojet uses an inertial drum, which similates load for a typical vehicle (3000-3500 lbs). This "typical" vehicle inertial maybe somewhat realistic for american muscle cars, but not for the typical import car.

With an eddy current dyno such as the Mustang dyno, the dyno operator enters the approximate weight of the vehicle, and with the Mustang dyno, an aerodynamic factor. The computer calculates the load to apply to the rollers based on vehicle speed to realistically load the vehicle. What does this mean? If you properly tune your car on an eddy current dyno, the car will run well in the real world.

Also, an eddy current dyno allows part throttle tuning. Part throttle can not be tuned properly on a Dynojet. Some will argue that part throttle tuning should be done on the street. However, street tuning can be dangerous.. operating a computer while driving. Also, how do you know what fuel/timing is best to optimize part throttle driving?

As for a comparison of Dynojet vs. Mustang numbers, I am currently compiling data to correlate the two. I will post results when I'm done. Here's a sneak peak: generally speaking, light vehicles (~2500 lbs) read 15% higher on a Dynojet. Heavy vehicles (~3500 lbs) read 5% higher on a Dynojet.

By Bob"

thought it might help