There are problems, and there are problems. I’ll let others speak to when your brakes would heat up enough to cause acute damage (probably just a few miles), but I downshift everyday heading into work for the first 1.5 miles of my trip. The speed limit is 35 mph there and the road has some decent grade. I’d drift up to 60+ mph if I didn’t downshift or ride the brakes. My brakes wouldn’t suddenly fail in this short distance, but they would certainly wear out a lot faster, and people behind me would have to see my brake lights on the whole time, which sort of defeats the purpose of brake lights. Thus, I downshift.
You’ll certainly wear out your brakes a little faster, but that doesn’t really count as “harm.” You’re not using your brakes non-stop, so you don’t have to worry about overheating them.
There’s not really any hard rule for when to downshift–it depends on the road, traffic conditions, etc. There’s a couple of one-block-long hills in my area that I downshift for–but they’re considerably steeper than anything you have.
If your car has “D-3-2-1” or something similar, as so many modern cars do, you could try putting it in 3, and see what happens. 55 mph is far too fast for 2, except in true emergencies.
The shifter on my mom’s car goes “PRNDB”. Took me a while to figure out what “B” stood for.
Seems to me the obvious approach is to link an inclinometer with a TPS: if the grade exceeds, say, 5%, the braking gear could be selected by the throttle position (how my car figures out how fast I want to go), upshifting as I press down on it. Does not address the “hunting” issue on climbs – though I suppose that could be worked in as well. And, of course, an off switch for the controller.
The other issue is how curvy the road is. The main road around here that I consistently use low gear on is the road to the top of Mount Greylock, which is about 7 miles with a average grade of overall average grade of about 6%, including a three-mile section with an average grade of 9% and some sections over 10%. Some of the hairpin turns have maximum speeds of 15 mph; god help anyone who tries to navigate them at a speed much over 35 mph.
For comparison, there’s a stretch of road on Highway 2 near here that has an average 5.6% grade over 4 miles. I don’t usually downshift on that one, though maybe I should.
I’ve rented an automatic in Europe. They’re available, just more expensive.
Thanks, all. I really have been wondering this for a while. You guys really have some HILLS!
I grew up in rural New England, with three miles of curving, unpaved one-lane road between us and the nearest pavement. The seasons could be identified by their driving hazards - Snow and ice, mud, dust, and fallen leaves. I’m very glad my automatic transmission still provided to option to shift down manually to gears 1 through 3 at need.
Same with Prius.
This. My town has a lot of long, slightly-inclined streets, with long distances between stop signs/traffic lights, along with 25-30 MPH speed limits. On some of these streets, if I’m traveling in the “downhill” direction, with my transmission in “D”, I can easily accelerate to 35+ MPH without touching my gas pedal. And, frankly, it’s extremely tiring and uncomfortable to lightly ride my brakes all the way down these “hills”, in some cases for almost a mile at a time. Not to mention that, if I have to stop suddenly, the driver behind me gets no warning because my brake lights have already been lit up the whole time.
It’s much easier to shift to “2”, which does a good job of holding my speed down to 30 MPH.
<nitpick>The Tesla Model S has a 1-speed gearbox. A fuel-burning car could conceivably be built with just one speed, but it would have crappy acceleration and efficiency.</nitpick>
I see a lot of people using 2 (aka L), but do people really use 1 all that often?
Doubt it. The last time I saw anyone use it was my dad, in a Land Cruiser, driving up a sand dune while towing a boat. While also in low range and four-wheel-drive. In 1993.
For about 20 minutes.
Come to the mountains, it’s necessary.
When I lived in Pittsburgh, lower gears came in handy when driving snow-covered hills. When descending, they kept the brakes from overheating, and breaking on snow-covered hills always carries the risk of sliding. I tried to go down them as slowly as possible to prevent loss of control. I shudder to think of going down some of those hills in D before the plows got to them.
Applying the brakes is known as braking, not breaking. 
If you don’t brake, you break.
Well, sliding certainly carries the risk of breaking.
I have no idea if anyone is interested but European trucks are mostly automatic these days. They are not like car gearboxes though, but they have a conventional 12 speed gearbox and a computer operated set of servos to operate a dry clutch and change the gears.
They mostly have a manual override and it is possible to force a change up or down even in auto mode. On a down hill, the easy way is to use the cruise control, which will operate the exhaust brake as speed increases above what it is set at. Forcing a downshift makes the exhaust brake more effective.
And in some cases, you break it, you’ve bought it.