Is it safe to cruise a manual gear car downhill with the engine off?
No matter how tasty this idea may sound, never, EVER try it!
Most modern cars are now loaded with electronics. The brakes, the throttle and steering are often controlled through the electronic microprocessor chip that is powered by the battery and/or the running engine.
Assuming that you turn off the engine, the steering or the brakes can give up. Why? These functions are usually done through hydraulic oils which are pressurized through pumps. These pumps work with the engine running to help them continuously run and maintain optimum fluid pressure for effective performance.
Even if the car in an old classic without any electronics bothering the real drive, do care that coasting will lead to wearing out of the brakes quite a bit. It is easier to downshift and let the engine retard the vehicle than letting the brakes do all the job themselves.