The responses you have gotten so far are good ones.
If you follow any automotive forums, or listen to Car Talk, almost every week you will hear some idiot say proudly that he downshifts instead of using the brakes whenever he comes to a stop light, for example. He is proud of himself for saving the brakes, but the expert response is always just what your brother-in-law said - "brakes are cheaper than transmissions". In other words, don't use engine braking for routine stops.
But your brother-in-law is wrong when he applies this stock answer to a very different situation - keeping your speed down on a steep downgrade with 2 tons of metal pushing on your back bumper. If you use the brakes for this, it is very possible that your brakes will overheat and fade - meaning they won't brake any more. And that means you crash. And guess what? Transmissions are cheaper than crashed vehicles and lost lives.
The secret is to downshift right from the beginning. Don't allow your speed to build up, and then drop your transmission down a gear. The shock of that downshift is indeed very hard on the tranny. Instead, just before you start down, let your speed drop, ease off on the gas, and downshift while your speed is still moderate. There is much less shock and your tranny will thank you for it. If your speed builds up a little while you are on the downgrade, use your brakes a little to bring it back down. If you find yourself having to use your brakes continuously, you have no choice but to downshift again, and sure enough, that will be hard on the tranny. But what you CANNOT do is ride your brakes - that is the invitation to disaster.
Ray had a real good post on this a while back - I'll see if I can find it.
Bill
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