So it sounds like you changed from a 100 mph to a 120 mph speedo.
Was the 100 speedo accurate?
If so the 120 speedo may need to be internally calibrated.
You can verify this by driving a known distance. The longer the better! Recording the odometer at start and stop. If the miles are very close to accurate (ie not 25% off) then the speedo gear is correct for you rear gear ratio and the tire height.
If on the other hand the miles are off by 25% then the 100 speedo was out of calibration and the 120 is more accurate. Change the speedo gear to correct the miles to be accurate and THEN if the Speedo is off have the Speedo calibrated. Or live with it.