What is happens to the brakes when an automatic drives below the speed for 1st gear with no added gas?

Question

In a manual transmission car, when a driver wants to go below the speed the car travels at in first gear with no added gas using the gas pedal, my first driving instructor told me you ease off the clutch, reducing the amount of power from the engine transferred to the axle. [update - several people have since told me that this isn't a good practice, and it's not the only thing he taught me that has turned out to not be a great idea...]

In an automatic car, to achieve the same effect, you push the foot brake. This bugs me for two reasons:

Is this constantly grinding the brakes? I'm in a city with bad traffic where I'm often crawling at 3mph with my foot on the foot brake. Should I be worried about excessive brake pad wear compared to what I'd expect in a manual?
At first I couldn't understand why the car wasn't stalling - I now understand that automatic transmissions use fluid rather than the contact friction of clutches which allows slippage. This makes me wonder if what is actually happening is similar to a manual in that less power is transferred to the axle and it's not actually the brakes that are taking the strain?

Is driving at such slow speeds in an automatic putting extra strain on the brake pads compared to what a manual would be doing, and if not, why not?

dlu · Accepted Answer

One thing to keep in mind here is that you're looking at the same problem either way – with a manual you're wearing the friction materials in the clutch, with an automatic the wear is happening to the brakes. But either way you're converting some of the excess energy into heat and friction. Obviously this is something that cars have to be designed to handle.

On the whole I'd say that wearing the brakes on the automatic is the lesser of two "evils" since it is usually a simpler and less expensive job to replace the brake pads than to replace the clutch. That said we've got two Jetta wagons that spend way too much time in traffic. Both are pushing 300,000 miles. One is an automatic, the other a manual. Neither is showing significant wear on the brakes or clutch (the automatic is pushing 80,000 miles on the front brakes, the manual has at least 75,000 miles on the clutch).

The durability of cars is amazing when you really think about it – even the ones that we think of as awful.

Pᴀᴜʟsᴛᴇʀ2 · Answer

This is what brakes were designed to do, either slow or stop a vehicle. It doesn't matter if it's a manual or automatic transmission. This will incur some wear, but really it is just minor in the grand scheme of things. Slowing you down at slow speeds does not create any appreciable amount of frictional heat, but it does generate some. If I were to guess, stopping a car at full tilt from 60mph is going to generate more wear on the brakes in one single incident than slowing your vehicle as you've suggested would create in several months.

Answered by Pᴀᴜʟsᴛᴇʀ2 on July 16, 2021

Harper - Reinstate Monica · Answer

It isn't really possible or sustainable for a manual transmission car to move so slow that the engine would bog in first gear.  In traffic, you have to "game" that by stopping, starting, coasting, rinse wash repeat.  You try to spread out those intervals as far as you can (without opening distance so much that other cars change into your lane).
With an automatic, yeah, you just drag the brakes.  This is no big deal, because the energy being dissipated is tiny compared to the capacity of the brakes.

What is happens to the brakes when an automatic drives below the speed for 1st gear with no added gas?

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