Once you change the gears in your differential, you need a new speedometer gear to recalibrate your speedometer. They're inexpensive, and only take a few minutes to change. These Recalibration Gears fit 1979-1998 Mustangs 23tooth$15.95 Select transmission type: ManualAutomatic www.realspeedauto.com Ph.631-563-7325 All parts listed are for the Ford Mustang unless otherwise noted. Copyright 2018 Realspeed Automotive Inc. All rights reserved.