I understand that shifting to a lower gear while going down a steep hill saves brake pad wear.However does it not increase engine and transmission wear as the engine rpm dramatically increases thereby negating any savings on brake pads?
Even though the engine rpm increases (it should not be dramatic) there is no more fuel being used. When the throttle is closed, the engine gets only enough fuel to maintain idle, regardless of rpm, so it’s more economical to use a lower gear to descend a steep hill. Those of us who live in hill country do it all the time.
The engine and transmission do in fact turn more in lower gears, so there is some additional wear to these components because of downshifting. You’d be hard pressed to measure this wear, though. Properly cared for, the wear items, which would be the piston rings, crankshaft and camshaft bearings and transmission gears and bearings should last well into several hundred thousand miles. This presumes also, of course, that you never allow the engine to over rev.
Note that this is only if the downshifting is done properly. “Properly” means double clutching and matching engine to transmission speed, and letting the clutch out pretty quickly without causing the car to jerk. If you just go from a higher gear directly to a lower one, then ease the clutch out to slow down you’ll go through a clutch in no time. Bear in mind also, that the double clutching technique typically requires a throttle “blip” while in neutral, and as a result loses a little fuel economy.
So, you’d rather take a chance wearing out a $2000 engine or a $500 clutch than a set of $50 brake pads? What’s the point here? Sure, if it’s a really long hill (like a mountain pass) and you are at risk of overheating the brakes, downshift and go down slowly, but in routine up and down driving use the brakes. That’s what they are for. They are supposed to wear out and you are supposed to replace them.
Just because engine RPM increases, that doesn’t mean that you’re applying the same amount of load to the motor as you would be if you were holding the motor at that RPM in other conditions. As mcparadise said, when you are descending in a low gear, the throttle is closed or barely open.
The combustion that’s taking place when you’re descending generates next to no power, and all of the “compression braking” that is occurring is derived from the engine trying to compress raw air. The load of the engine trying to compress air with virtually no combustion is FAR less than the load applied when the pistons are being forced down due entirely to combustion.
In short, coasting down a hill at 3500 RPM is far, far easier on the motor than going up the hill at 3500 RPM. The motor and transmission don’t mind, and the only way you could cause significant wear would be if you over-revved the motor.
EDIT: For what it’s worth, I’ll add that I’m not advocating downshifting and crawling down every hill and hump in the road. Out here in the west, we have all sorts of grades that require smart braking and gear selection, or you WILL burn up your brakes, and you WILL end up in the gravel run-away lane.
It is generally recommended to normally use the brakes to slow and stop the car. However on l-o-n-g downgrades use engine braking. Brakes can and do overheat and when they do, they stop functioning. That is far worse than any wear on the engine - drive train.
with FWD be careful doing this in the winter…increased resistance in the front end can result in the back end of the car to spin.
Timberwolf; this is neither a fuel economy or wear issue; it’s a safety and common sense issue. Any of us who live near or in mountains, do this instinctively. A long uphill drive needs a lower gear to put less strain on the engine, while a very long downhill will cook your brakes in high and give you less control if the surface is icy. Use a lower gaer as well. Try Pikes Peak, CO or the Highway to the Sun in Glacier National Park (closed in winter), and the need will become obvious.
I agree with Doc that this is not a wear issue but rather a safety issue. The car is slowed via either the engine of the brakes converting the energy into heat.
If you use your gears the energy is converted into heat in the compressed gasses in the cylinder that are slowing the car and is dissipated by the cooling system and expelled out the exhaust.
If you use the brakes the heat is in the brakes. That heat can cause the brake fluid to boil or even warp a rotor. In the old days of drum brakes, loss of brakes due to fluid boiling was not unknown. Drum brakes do not dissipate heat well, as the frictional surfaces where the heat happens are enclosed. While discs dissipate heat far better, overheating is still possible in extreme conditions.
I choose to not knowingly and unnecessarily create a condition that has even a slight chance of leading to loss of brakes. I choose life. I choose to use my gears.