Also I’ve noticed a number of people haven’t the foggiest idea what a tach is, and how it can be useful if you manually shift gears.
Manual transmission guy speaking here: I drive a 5-speed 2007 Mustang GT. Nice little entry-level “performance” car.
Typically, you want the lower gears for either speed limiting (going downhill without wearing out your brakes) or for passing. I’ll discuss snowy driving as well (I live and drive in the upper midwest, where snow, ice and sleet are common in winter months.)
If you have a long downhill stretch (mountains) it may be better to keep the car in a low gear (probably 2nd) to ease the load on your brakes. This is more applicable for semi trucks - they’re huge, heavy, and they can absolutely ruin their brakes and lose control if they don’t downshift on long hills. But most cars can handle the braking necessary on long downhill stretches, unless you’re doing it every day, repeatedly. But it is helpful. I do have one steep hill in my normal driving area; when I’m going down it it’s nice to just put the car in 2nd to have more speed control and not have to stand on the brake pedal.
3rd gear on most cars is a good “passing” gear - meaning, you’re on the highway, you want to pass someone. 3rd gear is good for getting reasonably quick acceleration without putting your tach into the redline. I do this all the time. The difference here between manual and auto trannys is that in all the autos I’ve driven, you shift to the left (passing) lane, floor it, and whatever controls the auto takes a few seconds to realize you really want to go fast. In a manual, you just shift and go.
As for snow/ice - and I say this driving a RWD Mustang near Chicago (So I am familiar with driving in snow with a car totally unsuited for such a thing):
If you get stuck on a slippery area, having control of the gear is very helpful. An earlier poster suggested that 2nd is the gear you want; I disagree, at least for my vehicle. What you really need to do to get moving on snow or ice is first: Disable traction control. Traction control kills power to the wheels if it senses that they’re slipping. Traction control is a great safety feature when the car is in motion; it’s terrible for getting the car moving.
Personally, I’ve had the best luck putting it in 1st with traction control off, and giving it a good amount of throttle. Often you end up going forward a bit, then losing traction and spinning the wheels. Then you want to put it in reverse, and again, give it a good amount of throttle. I usually end up rocking back and forth a bit, but I’ve never gotten stuck. Just keep rocking back and forth until you get the momentum to keep going.
Once you’re moving on snow and ice, absolutely turn traction control back on. It really is a good safety feature, it just makes it harder to get started.
Traction control? Fancy pants car. 
For the vehicles I’ve had, in slippery situations, both snow and rain, personally, I’ve found it easier to start from second, as it’s really easy to spin the tires in first. I generally try not to do the rocking-back-and-forth action, as it tends to make a bad situation worse. I try to slowly ease out and get traction, if I can. Admittedly, sometimes, you just gotta rock it. If you look it up, starting in a higher gear for slippery situations is fairly standard advice. YMMV.
Looking it up, it does seem that in a lot of automatics these days, 2 means “second gear only” and not “first and second.” I could swear my in my 80s and 90s era automatics, “2” meant limiting the gears to first and second, but perhaps I was not as observant back then as I am now, as I didn’t drive sticks until the late 90s.
Years ago, this was the case with Fords and was very handy in snow or on ice. But it was not the case with GMs or Chryslers, and isn’t now. I don’t know whether Ford stuck with it.
To my knowledge, gear “2” on automatics means the max gear the transmission will choose is 2, it may also choose 1. To those people who say it will only choose 2 - can you get your car to shudder/stall on 2? Or, launch the car in 1 and immediately shift to 3. Does the car change gears when you shift?
So, as people have said, it limits the gears your transmission will choose, for:
- more power (climbing)
- engine braking (downhill), also if your brakes don’t work
- traction
Something nobody has mentioned: if you want the fastest acceleration, you should stay in lower gears longer before shifting, but cars normally don’t do that because it wastes fuel. So you can “manually” upshift using the gears on an automatic to accelerate faster.
The interesting thing is - what was the main reason to have those gears on automatics - performance or safety? If engine braking required higher gears, would automatic transmissions limit the MINIMUM gear, or still the maximum?
Cars are continually getting better and more intelligent with their shifting patterns. I know some brands that have a “snow” setting for the auto, that will start it in second.
On top of this, the way that an automatic works is inherently more gentle than a manual.
But I would also need to point out, in my auto if I mash it for max acceleration, the car will hold the gear until redline before shifting - so the short shifting you are referring to may not neccessarily be true.
Ok the instructions for the 2011 Honda Accord say 2 locks the transmission, but D3 may select lower gears. 2011 Toyota Camry says it’ll select that and lower gears. I guess second gear is powerful enough to do without first.
Automatics will roll forward in D without your foot on the accelerator, it’s called “idle speed” or something of the sort. In many automatic cars that speed can be faster than 5 mph. In other words, you have to ride the brake in D to slow it down to 5 mph. Putting it into a lower gear gives it a lower idle speed and eliminates the need to ride the brake.
Why is it done that way? Haven’t the foggiest notion, but I can always spot someone who’s only driving experience is with manuals because they don’t know about that quirk and/or are surprised by the car wanting to creep/roll forward in D without input from them.
I do know about riding the brake in automatics. But wouldn’t you be in first gear off the line whether you’re in D or 1 on an automatic? This is what I’m wondering about. Plus, off the line, the lower gear should accelerate faster to 5 mph than the higher one, anyway, if D starts you off in anything but first. I guess I could see that if the auto switches to second by 5 mph, it should eventually coast faster, but is the shift point that low?
It’s cheaper to get new brakes than a new engine. Is it definite that long downhill stretches of engine braking don’t decrease the life of your engine?
Honestly… I don’t know. My current car has a computer that, when it comes to driving matters, is smarter than I am. I don’t know how it was programmed but these days either alternative is possible.
But you need to get new brakes while you own the car, a new engine will most likely be someone else’s problem.
No it does not.
No. “engine braking” means using the pumping losses of the engine trying to draw air through a closed throttle combined with a lower gear ratio to slow the car. It does not cause any additional wear in the engine of any consequence.
It won’t be cheaper if your brake fluid boils and you can’t stop when needed.
Your engine is likely to last for a few thousand hours of run time. Over the life of your vehicle, deliberate engine braking is not likely to compose more than an hour of that time. Moreover, engine braking puts less load on the engine than highway cruise, and with lower temperatures (since little or no fuel is being burned in the combustion chambers. Wear due to engine braking will be negligible.
If you are descending a miles-long 9% mountain grade with a car full of meat and/or cargo, using the friction brakes to decelerate from 80 MPH down to 60 MPH three times per minute for a few minutes stands a fair chance of cooking your brakes. You may warp the rotors, you may experience reduced braking capacity due to brake fade (pad friction coefficient reduces at very high temperature) or in severe cases (especially with moisture-contaminated brake fluid) you may boil the brake fluid and lose braking ability altogether.
Descending a one-block-long hill in San Francisco at 20 MPH? Don’t bother downshifting your automatic transmission; your friction brakes will do just fine here.
Automatic transmissions will shortshift if you’re light on the throttle - but if you’ve got your foot to the floor, the computer will not shift until RPM’s are at or near the redline; no need to use the selector to delay the shift.
If the transmission is not in neutral, the car wants to creep forward until the engine is idling at the same speed it would idle at in neutral[sup]*[/sup]. If you let it upshift (i.e. the selector is in “2” or “D”), then once the car gets going fast enough, it will shift up to a higher gear and end up idling forward faster than if you had held it down in first gear. Listen/feel carefully (especially if you’ve got a tachometer on the display), and you can probably tell where the shift is.
[sup]*[/sup]late-model vehicles may open the throttle slightly when you put the transmission in gear so as to avoid having the RPM’s drop precipitously, so if you let it roll forward at its desired speed, the RPM may end up being higher than you would see if you were stopped with the selector in “P” or “N”.
To answer the question about cars that 2nd means 2nd only. There is no problem with the engine studdering, it appears the car is just as happy starting out from a stop in 2nd as 1st, though with less acceleration.
Also some automatic cars with a ‘manumatic’ mode, meaning you control the shifting of a automatic transmission by moving a shifter to up/downshift - autoshifting doesn’t happen, allow the start from a stop in 2nd by using the manumatic mode. Subaru is one such manufacturer, at least some of their older models, where to start out in 2nd you would have to use the manumatic shifter to change from 1 to 2 at the stop, then you start out in 2.
Hell, with some of the really torquey cars, you can even start in 3! (My friend’s 2012 Camaro is a manumatic with paddle shifters, and I was surprised that it would let me select 3 to start in manual mode. But not 4. )
It is even more expensive to haul the remains of your car out of the ravine* and and embalm and bury your corpse after you boil your brake fluid and fail to make that last hard turn on a mountain grade. Yes, brakes are far better now than they used to be, assuming they are maintained. When was your brake fluid last replaced? How much lining do you have left on your pads? Are your brake hoses in good condition or are they getting spongy?
Engine braking** lightly** loads the back sides of gear teeth, and the opposite half of journal bearings that see very little wear otherwise. It is at fairly high RPM giving full oil pressure and low forces, so wear is virtually nil. Yes it adds some minute wear, but it is wear to areas that won’t wear out anyway, and will still be fine long past the point where wear from normal driving requires replacement or rebuild of components. Many modern engine control units completely stop the flow of fuel under these conditions, so it can actually use less fuel than riding the brakes.
It is absolutely false to say that brakes were designed to control speed and transmissions/torque converters/clutches/etc. were not. The existence of options other than “D” betrays the fact that the designers envisioned valid uses of those selections.
*expensive enough that they often just leave the dead cars to rust away.