The correction box will work if your speedo is off a percentage. For example: 30% off at 100 (30kph), 30% off at 50 (15kph). That is usually caused by fitting larger tires than the stock 12" tires. Or if you have changed the diff ratio.

It won't help if it is off a fixed 30 kph. For that you need to fix the gauge by resetting the needle or swap the speedometer with a good one.

Click Speedometer, see section Speedometer Calibration.