Using friction brakes for long time can increase easily its temperature and lower vehicle brake performance in the downhill process. The drivers' hysteretic perception to future driving condition could mislead them to stop untimely the engine brake, and some other auxiliary braking devices are designed to increase the brake power for reduction of the friction brake torque. The decompression engine brake has complex structure and high cost, and the application of eddy current retarder or hydraulic retarder on the commercial vehicles is mainly limited to their cost and mass. In this paper, an innovative brake guidance system for commercial vehicles with coordinated friction brakes and engine brake is introduced to guide the drivers to minimize the use of the friction brakes on the downhill with consideration of future driving conditions, which is aimed at releasing the engine brake potential fully and controlling the friction brake temperature in safe range. The guidance system without the retarder installation and changes in vehicle structure would overcome above difficulties effectively, and can match various vehicles without basic parameter calibration at installation. The brake optimization algorithm with simple structure and fast computation speed is built to optimize the vehicle speed profiles before the vehicles reach the downhill. The downhill information can be obtained by geographic information system and global position system, and the terrain data are processed for more accurate downhill length and gradient. The vehicle state information from CAN (Controller Area Network) bus and the road gradient from the gradient sensor are used to measure the engine brake performance during transportation. Braking distances of the engine brake and the friction brakes are reasonably distributed to plan the optimal vehicle speed profiles which are used to guide the drivers to adjust the vehicle speed for releasing the engine brake potential fully in the downhill process. The simulations are carried out to validate the effectiveness of the brake guidance system. From the simulation results, maximum friction brake temperature reduction with the guidance from the brake guidance system is 23.9 degrees centigrade compared with uniform motions at different constant speeds under the simulation downhill condition with overall length 4800 meters and maximum altitude difference 134 meters. The friction brake abrasion and heat fade phenomena can be effectively reduced when the brake guidance system is popularized. The brake performance of commercial vehicles on the downhill can also be certainly enhanced.