The chassis dyno is a great tuning tool as was mentioned previously. I must qualify this by saying it needs to be a "loaded" chassis dyno that can simulate wind resistance and other factors. To me the numbers are meaningless UNLESS being compared to before and after modifications on the same dyno under the same conditions. There are just to many factors/settings that can impact the numbers output by the dyno. It is a tuning tool and nothing more. I say this as an owner/operator of a Mustang Chassis Dyno. It has saved me oodles of time (and money) that would normally have been spent going to the track and testing (actually sitting in the staging lanes mostly).

And for reference my car weighs 3700#'s, and on street tires through the mufflers goes 10.5@126 N/A. On my dyno the car makes 439hp to the wheels.