Ok, I'm not sure what you are talking about here regarding the readings. Are you referring to the dyno readings and setup or the hand held that he used (and I got as a part of the tune - Predator by Diablosport)

 

They are somewhere around 8-12% lower on average.

 

But if it's a Mustang dyno then it requires that the weight of the car be input to ge ta proper reading. One time I had the SS dynoed and the operator put in 3800 lbs. and the car only weighs 3380lbs so that skews the numbers.

Dyno numbers are just for bragging rights. Track MPH is the proof.

 

That's the operator's fault, then. Not the fault of the dyno.

I agree that the track MPH really shows how the car performs. There's no point if your car only makes power for 1500 usable rpm, but puts down high numbers.

 

Here is what the guy who did my performance tune responded to my low RWHP readings:

The RWHP for YOUR car is accurate. The numbers from everyone else that you are compairing to where most likely dyno'd on a Dynojet. Dynojet dyno's are known to read high. Which is evident by a stock Z's RWHP being at 325 when they were not even rated at that level from the factory at the flywheel. Our Mustang dyno is more of an accurate reading. The biggest thing you need to look at is the difference between the base line run and the "performance tune" run.
We tested a 98 LS1 firebird that was bone stock (rated at 305 flywheel HP). It dyno'd at 230 here. That same car, same day on a Dynojet dyno'd at 292. I doubt the drivetrain loss was 13 hp.