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9 The speedometer cable is disconnected next. Be prepared to change the speedometer drive gear, especially if you're doing a rear end gear swap as well. If your Mustang has 2.79:1 or 3.00:1 rear end gears, overdrive will bog down the engine because rpm drops way below the engine's torque curve. Best axle ratios for the AOD are 3.55:1 (best for cruising), 3.91:1, or 4.11:1.