In order to determine the percentage error your speedometer is reading in order to correct it, the easiest way is to visit this online tire comparison calculator.

Enter Size 1 as the Factory Stock tire Diameter

Enter Size 2 is your current custom tire size (radial)

Do the math using your actual GPS reading at 60mph (or your speed of choice) and what your mounted speedometer needle is pointing to.   It is important to enter the numbers in the correct order to get the correct ratio % reduction box.