Chast,
You ask whether it is better, when going down a long grade, to:
A- wear out the brakes or
B- put extra strain on the transmission.
With those choices, it would seem like a no-brainer ... anyone would prefer to have to do a brake job than a transmission job, but ...
The real question should be, is it better, when going down a long grade, to:
A- use my brakes all the time and take the risk that they will overheat and fail and I won't live long enough to have a brake job
B- downshift with the transmission as it is designed to do.
If you look at it that way it's also a no-brainer ... but the answer's different.
I tended to use my brakes too much and had them start pulsating on me on a steep grade. I was lucky because the grade wasn't too long. I ran out of hill before running out of brakes. I never want to go through that again. Once the problem starts, it's too late to downshift.