Just take the car to the drag strip and get a trap speed.
MPH doesn't change much regardless of your time. Time is more related to how well you start rather than power.
Use the weight of your car and mph to calculate your power. Correct the numbers to sea level and you have a reasonable comparison to others or your own car on diffrent days or after changes.
http://www.prosystemsracing.com/calculate.html
http://www.modulardepot.com/density.php