I don't think down-shifting our diesels is a good idea. Holding the speed on a downgrade is a different matter, though.

Back in the '50s and '60s everyone racing sports cars downshifted. If you didn't, the brakes wouldn't last the race. More modern brake materials have changed all that. Most racers now only downshift to the gear they need exiting the corner rather than using the engine to slow the car. Brake pads and linings are definitely cheaper than rebuilding engines or transmissions.

Just my $.02,

Wes