its easy.... takes 2 minutes once you find out how many MPH you are off. PP manual explains it better, but here is what i know.
Get/borrow a GPS, set your PP at 20 mph and see how off your speed is from the GPS. in your case you may be +3 or 4. from there you stop, turn off your PP, and go to the menu and calibrate the speed by -3 or however much you were off.
less than 5 minutes total time and you are done.