A transmission controller implements a neutral idle feature to reduce fuel consumption. A brake controller implements a Hill Start Assist feature to prevent roll-back when a vehicle is launching on an uphill grade. The transmission controller and the brake controller communicate to implement these features in a synergistic manner. Within certain road grade ranges, the transmission requests Hill Start Assist before activating neutral idle and does not activate neutral idle until it receives confirmation that Hill Start Assist is active. The transmission controller provides a powertrain torque estimate to the brake controller which the brake controller uses to determine when to release the brakes during an assisted start.