I am not a Dyno Jet operator but I have had several customers putting their cars on them. My experience is that on chassis dynos at least of the rolling inertia wheel variety, that there can be no reasonable comparison to real world going down the track rear wheel horsepower is.
I think that a lot of this is becuase cars with any power and racing torque convertors are just blowing through the convertor. I see no other way to explain losing several hundred hp away from what was on engine dyno or for ending up with some outrageous horsepower curves that are far different.
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I see much better hp numbers in essentially the same engine combinations when the car is sporting a clutch and manual gear box.
Another huge probloem in my mind is the way they weight the wheel. The only way it an be close is if your vehicle is for what the wheel is weighted for.
So in summation, for me rear wheelo horsepower on a chassis dyno is used as a standard only and when used again can show results of changes only. The actual numbers are not real as compared to going down the track.
Most chassis dyno operators will tell you that the gearing of the car does not make much difference. I know better. Like was earlier mentioned, it does play a major part in how quick you get the rollers to moving quicker.
There is no set percentage of loos when using an automatic tranny because each convertor all by it's self will show diferent results. The old school high stall torque convertors that use the reverse pitch technology really suck.
Ed