[dad911]

In a panic stop, the limiting factor is the contact of the tire to the road. In any modern car, the brakes are strong enough to 'lock up the tires' therefore engine braking is not necessary.

However, try going down a 3 mile descent in a tow vehicle with a full trailer, like the hill into town from Watkins Glen. Brakes alone, and they will be smoking & glowing by the bottom. However, shift into a lower gear, and you will hardly need the brakes at all.