11

In a car with an automatic transmission, we are normally greeted by the acronym PRNDL, or in some cases (older cars) PRND21.

I know that the L, 1, or 2 will cause the transmission to stay in a (L)ow gear and not upshift, but I've heard various explanations of what the low gears are for. I've only used them to play around with, and never felt like I had a reason to use them. It's the automatic's job to pick the correct gear, right?

PRND21

  • In the case of the Toyota Prius, which has a eCVT transmission, the B (which stands for engine Brake) takes over the job of the L gear. It is quite self explaining. Under this mode the Prius will keep the gas engine running to serve as a engine brake. – Gabriel Diego May 4 '16 at 0:13
  • You Must learn this in West Virginia or you will have Problems ! – Wes Miller Feb 15 at 14:23
14

To go down hill

By setting L (or 1 or 2), the gear will stay low and you will be able to use engine brake, instead of using brakes all the way down the hill and suffering from fading. The transmission will not necessarily pick a lower gear when going downhill, although they will pick a low gear if you are going uphill.

Never brake the car for an extended time when going downhill. This can be a life or death decision, specially if you are crossing a mountain pass. If you use the brakes for a extended time, besides needing to maintain them more often, you may risk having no brakes after a while. Truck drivers should always do that and often there are sign on the side of the highways remembering to do that. Usually those signs read "Use low gear". For trucks that don't do that or that suffered a brake failure for other reasons, there are escape ramps filled with sand or other material to reduce the speed of runaway trucks.

I personally always use L whenever is safe to do so in order to save fuel and the brake pads. It is a habit that I kept since I started driving (in a stick shift).

  • 1
    Great point, totally forgot I do that too! – MooseLucifer May 3 '16 at 19:57
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    Some transmissions when you choose 2, it will start out in 2 from a stop, which is nice for icy conditions. – Moab May 3 '16 at 22:20
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    A note on overusing engine braking over using the brakes (which I do): new brake pads are cheaper than new transmissions. – Separatrix May 4 '16 at 10:46
  • @Separatrix: The wear on clutch and transmission during engine braking is not higher than during normal driving. (Unless you're doing it completely wrong, of course, letting the clutch slide or somesuch.) And even if the wear would be (marginally...) higher, that's better than finding yourself without grip in the brakes at the end of a serpentine, because you can't fit new brake pads right then and there. – DevSolar May 4 '16 at 11:09
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    I have found that my 2016 Dodge Caravan will downshift when going downhill and the cruise control is set to try to control the speed (although it can't keep it exact). – BPugh May 4 '16 at 14:40
9

Two additional uses alongside engine braking controlling downhill descents:

Low gear, high revs uphill on loose ground like sand

If you need to go up a steep hill on loose terrain like a sandy dirty track, dune or fine gravel, you need to build up momentum before hitting the incline and you need to put your foot down and get high revs for the speed you're travelling at. Leaving an automatic on D risks it going up a gear too quickly, having insufficient power and losing momentum on encountering the incline leaving you in stuck with wheel spin. Fixing the gear in 1 or L lets you put your foot down and get the high revs necessary.

I lost a tire in a remote African village attempting a dusty sandy hill in D then losing momentum like this. After changing wheel with some help from the villagers, then rolling all the way down to flat ground, I hit it in L with foot firmly down and cruised over the hill like it wasn't there.

From Land Rover's off road driving guide:

Deep mud or sand needs a steady momentum to carry you through ... in sand, the lower gear the better.

Higher gear, low revs on ice and mud

Too much power when driving in icy conditions or wet mud will simply cause the wheels to slip, so you might want to put the gear in to 2nd and then drive with low revs. In D, it might drop down a gear and you'd have too much power.

From Land Rover again:

In mud, too low a gear causes wheel spin.

More from the BBC

Stay in a higher gear for better control, and if it is slippery, in a manual car move off in a higher gear, rather than just using first.

In very bad conditions, crawling along in 2nd (and possibly even starting in second, depending on how the particular automatic transmission adapts) might be necessary.

7

The only time I've ever felt the need to use anything other than 'D' in an automatic is when towing or climbing a steep grade. That's not to say I don't play around like it's a stick shift sometimes, but as you said, it's not necessary.

The other day, I had to pull a damaged car (one wheel was locked up) across an apt complex. I put the tow car in 1 in order to keep the torque and speed up, instead of allowing it to shift into second and dropping the engine out of the power band.

Similarly, when climbing a steep highway grade in an under-powered 4 cylinder car, I left it in 3, because it kept wanting to shift into 4th, then it wouldn't have enough torque to hold 70 mph, so I'd have my foot to the floor and it would downshift into 3rd to get the speed back, and so on and so forth.

Edit: gabrieldiego's answer reminded me I do that too, having gotten so used to driving manual transmissions. Because the torque converter is not a direct link to the engine, however, it doesn't slow you down nearly as much as engine braking with a manual transmission.

  • That an unusual usage for the L. Most of the time I would simply apply gas to downshift when going uphill. Many newer auto transmissions will keep a lower gear under constant load, but of course, the machine may fail sometimes. – Gabriel Diego May 3 '16 at 20:01
  • @gabrieldiego I didn't use L, nor did I used 3, actually. I used the O/D OFF button to hold it in 3rd. If I left it in D, I'd floor it and the car would shift into 3rd, get up to speed, then I'd have to back off the throttle and it would shift back into 4th, where it didn't have the torque to maintain the speed, so I'd floor it, it would shift into 3rd to get back up to speed, then I'd back off the throttle and it would shift into 4th, so I'd floor it..... and so on and so forth. – MooseLucifer May 3 '16 at 20:06
  • That's what I meant. Many modern cars (not all) are able to keep the gear if the load is constant, but it looks it was not your case. – Gabriel Diego May 3 '16 at 20:09
  • @gabrieldiego yeah, just the sad reality of driving an under powered car :c – MooseLucifer May 3 '16 at 20:12
  • I always drove under powered cars and never felt I need to more power. Even a car with an 1.0 liter engine with 70 hp can maintain freeway speed limits even when fully loaded. I even got a number of speeding tickets in such a car. Of course that a stick shift is way better to control such tiny power. And also it is not the case if you can afford the tickets or drive in the racetracks :-P – Gabriel Diego May 3 '16 at 20:17
5

While the others has stated why you might want to use one of the lower gears, they aren't explaining what happens when you actually use the lower gears.

On most vehicles with automatic transmissions, when you put the vehicle into "2", the transmission goes into second gear. You don't start in first gear, then shift up to second, it starts in 2nd gear. This reduces the amount of torque going to the wheels, which may help in situations where you want to reduce wheel spin, such as when stuck in the mud/snow. Having the wheels turn slower actually helps in this situation, as spinning most of the times will just get you stuck worse.

  • I'm a little confused by this. I've only driven automatics after passing my test, and only the Renault Clio II had this feature - but it was via a dedicated "snow" button which started you off in 2nd. In all my other cars, the manual has always said that the selector limits the highest gear, rather than starting you in that gear. – Virtual Anomaly May 4 '16 at 12:05

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