Correct, but also when pads overheat they lose much of their stopping ability, and when drums overheat they expand (to the point where the shoes won’t contact them).
You, sir, are wallowing in understatement. 
I was under the impression that when the brakes fail due to overheating, it is generally because the brake fluid boils. Is that incorrect?
"811.495 Unlawful coasting on downgrade; exception; penalty. (1) A person commits the offense of unlawful coasting on a downgrade if the person is the driver of a vehicle on a downgrade and the person coasts with the gears or transmission of the motor vehicle in neutral or with the clutch disengaged.
…
(3) The offense described in this section, unlawful coasting on a downgrade, is a Class D traffic violation. [1983 c.338 §656; 1985 c.16 §321; 1995 c.383 §76]"
(You can debate wear-and-tear and stuff all you want. But if you’re coming down Cabbage Hill on I-84 towards Pendleton, surviving is the #1 priority.)
I’ll just bump this with a note that I have had my brake fluid boil and yes, the brake pedal swings smoothly to the floor like it’s not connected to anything. Very disconcerting.
Out here in the mountainous regions, only using your brakes for the big long downgrades is likely to lead to your introduction to the “runaway truck ramps” that lead off the roads.
For those who haven’t driven in the mountains before, a runaway ramp is a big long, uhh, well, ramp that exits nearly parallel to the road and heads up the mountainside. It’s usually paved very deeply in fine gravel.
The idea is when you, the Semi-truck driver, have a brake failure due to overheating, you can swing off the road at 90 miles an hour, hit the gravel ramp, and head up the mountain. The slope and the friction of driving your wheels through that gravel is suppose to stop the truck before you reach the end of the ramp.
Like this (without the loop).
It’s not uncommon to drive past one of these and see multiple deep ruts in the gravel. Using a lower gear, even on a passenger car, can be the only way to prevent this from happening.
Another data point, the road up and down Pike’s Peak is, not surprisingly, very steep. There’s a booth at the bottom of it, checking the leaving cars, where a guy with an infrared thermometer checks your brake temp and waves you to a parking area if they’re too hot. I guess they lost too many tourists to failing brakes.
I don’t know how much of it is fluid boiling and how much is friction components overheating, but when you descend Pike’s Peak, every so often there’s a checkpoint where you must stop while a ranger checks your brakes. He measures the temperature of the the front rotors. (If they’re too hot, you pull over and wait until he allows you to continue.)
The emergency brakes are just your rear brakes. If they’re already overheated when the master cylinder engages them, they’ll still be overheated when the manually-activated cable engages them.
There is no savings, nor advantage in freewheeling. It does not save fuel. It causes excess wear on certain bearings in the drive train. It can be dangerous.
There is no good reason to do it, unless you’ve just run out of fuel and can coast into a nearby station, or lost power and can get to a place where you can get your car off the highway and out of danger.
Yes, but the problem with brakes overheating is largely in the hydraulic system. I have a hard time believing that the pads and the hydraulics are likely to fail completely, simultaneously, and without any warning.
Seems to me the best approach – whether you engine brake or not – is to be alert for fading brakes and pull over and stop as necessary.
I think that if you want to improve your economy on a downhill you should not put in your clutch but rather just take your foot off the accelerator. The reason for this is if the clutch is in you still need to use fuel to keep the engine turning but if the clutch is out then then tyre rotation will keep the engine turning and no fuel will be used. It works like this on my car and saves me about 0.1 litres/100km (When I say the clutch is in I mean that the pedal is depressed)
I tend to disagree. When in gear, the engine is applying braking force. On all but the steepest grades, the weight of the car is insufficient to overcome that braking force, so the accelerator must be pressed to keep up to speed. BUT, on moderate grades, in neutral, the weight of the car is enough to keep up to speed. So it is possible to coast down most hills in neutral without pressing the accelerator at all, but impossible to do so in gear. Wouldn’t you agree that an idling engine uses less fuel than an engine with the throttle engaged?
So on very steep hills I would agree with you, but on most hills, no.
I used to do this on an eight mile long downgrade into my old home town, a long time ago. Inherited the trick from my dad. This was in the days when carburettors were still alive and kicking and engine braking actually used more fuel than driving down normally.
The heap that I was driving previously had no power steering or brake servo so there was no impact on the steering or brakes - I just turned the engine off and let it roll.
I had been used to doing this for quite a few years with no ill-effects.
Changed cars to a relatively new Citroen GS.
For anyone unfamiliar withe the bigger Citroens, they have a fairly sophisticated hydropneumatic suspension system fed from a central, engine driven high pressure pump and various fluid reservoirs.
The same system also feeds the brakes and power steering.
The GS however didn’t have power steering - didn’t need it. By far the best steering system I have encountered, far better than the newer models with power steering - but I digress.
As per usual, I shut the engine off at the top of the hill. After a couple of miles the speed was getting rather high for the conditions (80mph) so hit the brakes.
Nothing. Just…nothing. The suspensions systems flexing had used up all the reserves and there was nothing left for the brakes.
Used the handbrake (mechanical, separate set of pads on the front discs) to decrease the speed a bit while restarting the engine.
It was fortunate the there was no-one ambling down the hill in front of me at 20mph and taught me a lesson I will never forget.
My current Citroen, a C5, has the braking system separated from the remaining hydraulics. Probably a good thing.
According to Jeremy Clarkson and who am I to argue, idling does in fact use more fuel than using the engine to retard the motion of the car in preference to the braking system, as the EMS cuts off the fuel supply under overrun conditions.
Edit: messed up the link coding
Sorry for the sidetrack, but I would suggest putting the transmission in neutral rather than holding down the clutch, the reason being that holding the clutch in causes wear on the throw-out bearing. If you do this all the time, your throw-out bearing could fail prematurely.
If you are insisting on using only your brakes to control your speed on a steep/long downhill drive, you are unlikely to notice your brakes fading in time. Because you are using your brake-pedal all the time. You will notice that your brakes are getting overheated because you pushed the pedal to the bottom and your car is picking up speed. A few moments later your brakes will have failed completely, simultaneously and with little warning.
I will repeat what everybody in this thread has said/implied:
Use your engine to control/maintain your speed, use your brakes to decrease speed.
In extreme conditions it would be prudent to stop regulary to inspect your brakes for signs of overheating, you will notice very little from behind the steering wheel.
It will also drastically increase the wear on the clutch plate as it is not completely disengaged, just the pressure relieved sufficient for it to slip.
There are a lot of assertions flying around in this thread and I think both sides need to provide some cites. Just an observation from a third-party who is interested in finding out the straight dope here.
Anecdotal, but a buddy was a truck driver for a drilling company. He would coast in neutral down long grades but with 2 good reasons not to. On the Mack he was driving, you could not get the transmission back in gear over a certain speed, and if you just used the brakes you could run out of air in the system faster than the idling engine could build it back up. I was with him once, I have no idea how fast we were going but we had the speedometer buried for miles, passing cars like they were standing still. There was a climbing grade a few miles away that he used to slow down enough to get the transission back engaged (he evidently had done this on this stretch of road before) and I lived to tell the tale.
So in Texas it’s illegal to take your car out of gear to coast down a hill, a truck isn’t even supposed to coast with the clutch disengaged.
I don’t see why that would cause the pads to fail simultaneously. Anyway, I don’t speak for all drivers but I never use the brake pedal “all the time.” Finally, when you are coasting down a hill, you should be extremely alert for any indication of fading in the brakes.
It’s definitely illegal in California. I don’t even need to look it up, because I’ve read the statute many times. I thought it had been pretty well established that it’s illegal, the reason being that you don’t have as much control of your vehicle when it’s not in gear.
The discussion has branched off into not whether it’s legal, but whether it does any good.
I can find no such statute in the Ohio Revised Code.
My great aunt, who raised a big family in Canton, Ohio during WW2, once told me that because of gas rationing she would coast down any large hill in neutral whenever she could.