Is downshifting on hills harmful for non-trucks? [duplicate]

Question

Possible Duplicate:
Does Downshifting (Engine Braking) Cause Extra Wear and Tear?

Since the days of taking drivers ed (many years back). I was told that when you encounter a steep or extended downgrade, to conserve your brakes and help increase braking performance it's a good idea to shift from Drive to 2nd (at least on automatic transmissions which is all I've driven).

Still, while the advice seemes valid and for the most part it's helped greatly every so often in winter on hilly areas, on a recent trip through upstate New York, I encountered numerous rural hills so steep and long that when I downshifted, I was hitting 3-3.5k RPM peaking at 4k even as I was doing 40mph and breaking heavily (but steadily) to keep the RPMs in check.

Anyway my main question is that when you see those hills with the signs indicating trucks must downshift, is it also a good idea for people driving sedans and other non-commercial vehicles?

The main reason I ask is because I've heard downshifting saves brake pads but can damage the transmision, so the rationale is to avoid downshifting when possible. I've done this for much of my average driving however I definately would love to get a more solid opinion.

Answer 1

tl,dr: The short answer is no, with caveats.

The longer answer starts with "well, you can kill your car with anything if you try hard enough."

Let's use specific examples of when you should use your transmission for engine braking: Waimea Canyon or Mount Washington. In either case, you are descending thousands of vertical feet at significant grades. If you were to ride your brakes the entire way down, it's highly likely that the brakes would overheat and become less and less effective until they were essentially useless.

This would be followed by a loud crashing sound.

In fact, at the Mount Washington Auto Road, they will not let you proceed up or down the hill unless you are in "2" on an automatic transmission vehicle.

Caveat: when you intentionally put your car in a lower gear and drive down a steep grade, you are giving gravity control of your engine revs. As I say, it is sometimes important to use engine braking (we've discussed the benefits to fuel economy before). However, you are the driver of the vehicle so you have to make the call about how much is too much.

A simple rule of thumb: the redline is still the redline. If your engine braking is pushing the limits of the rated engine speed, it's time to use the brake pedal and slow down significantly and, possibly, drop down another gear. Otherwise, you're going to break your engine in an effort to save your brake pads.

Answer 2

I have a 1992 Honda Accord auto transmission with 333,000 miles. I have always downshifted one or two gears to save the brakes and avoid excessive speeds on downhill stretches. I often drive in areas with long hills, sometimes quite steep hills. Also often in flat urban driving will downshift a gear to let the car slow down when approaching a red light.

I have NEVER had transmission problems and have NEVER had any issue that required transmission work.

My mechanic agrees with my driving. His only caution is to do the downshift on hills early by planning ahead, and not to do it when doing so would approach the red line.

I have asked Honda about this question, but find it impossible to get any response from them.

Answer 3

There are some long 1 in 10 grades in Scotland where the advice for all drivers is to choose a low gear to avoid brake fade, as @BobCross points out.

There are slight differences between automatic and manual transmission cars, though - as you do get more control of revs with a manual, so make sure if driving an automatic that you change into 2 before the slope commences.

Answer 4

Great answers above, but as a general comment: if your going down a long steep hill and you don't do it that often (or if you're not familiar with the area) always use engine braking. You can fix a transmission if you somehow screw things up, but you're still much better of than if your brakes overheat and fail and you end up in an accident.