Dyno tests

plus if you are not using your intake/carb/jetting/plugs/hdrs/fuel delivery system/timing/mufflers it could be even more off once you get it in the car and running... makes it a bit of a joke doesn't it to spend $500 or whatever. Cheapest way is to run it down the track a few times and and go by your best/average mph on the slip, get the weight of the car with you in it at the line and use the Wallace calc, that will tell you how much hp you have on the day under those conditions, much closer than dyno hp. If you run it on street tyres you'll mph higher than with slicks so try and get the car to hook as best you can.

http://www.wallaceracing.com/accel-calc.php