On an automatic transmossion the gear selections are:
P R N D Low ( or 321 or whatever gears there are).
When driving, I never use low, nor does anyone else I’ve talked to. Its an AUTOMATIC! If I need to be in a low gear, the transmission will automatically shift into the optimum gear for whatever driving conditions are present.
Putting the gear in low only makes the car slower and the engine rpms faster. Why is there a low gear?
When I lived in Idaho, a state that seems more vertical than horizontal, I’d use low gear for those really steep parts of the road (like the pass from Jackson WY into Idaho). At some point, I could go faster in low gear than in D. Also helps in braking when going downhill.
Well. those of us who enjoy and understand driving drive stick shifts… them we don’t have to worry about things like this 
We are actually in control of the car all the time
Jeers aside, there are two situations when the low gears are advantageous:
When towing: Shifting into a higher gear bad when you need torque. Since an automatic is designed to shift when best for fuel economy, it will be confused when expected to stay in a low gear to provide torque… therefore, you have to use your brain to override it’s programmed logic.
When going down a long steep hill: Keeping your foot on the brake to maintain control will backfire… you will lose braking power as the brakes overheat and brake fluid begins to boil. As there is more rolling resistance in a low gear, it allows you to use what we stick-shifters call “engine braking”. Your engine’s inertia will provide control to your coasting.
Like you said before though, most people will never use anything but the Overdrive setting.
Besides, don’t you like the fact that you have at least some semblance of control over your transmission?
In some vehicles…specifically trucks they call it the “granny gear” and here’s my best understanding.
If you are on a steep incline, or slippery and/or rocky terrain, the lower gear gives you enough power without allowing engine to overcome the wheels on the terrain.
It’s like a walker on wheels. That gear is powerful enough to tow the load but doesn’t spin the tires too fast.
I could be wrong in explaining it but that’s the best way I can allow most to understand it. In most “cars” it’s not needed but in truck situation where you are carrying a heavy load up a steep incline it puts less strain on the engine.
I gather that’s why semis drive so slowly up mountain passes, usually in the lowest gear but enough power that pushes/pulls it up.
Friends have noted that fact that even if I’m driving an automatic, I still shift.
I prefer control over the car’s RPMs, because when taking a turn, I’d prefer to downshift into the turn, then upshift on the way out… not vice-versa as most automatic transmissions, in their “wisdom”, feel should occur.
But that’s me…
What Techchick is referring to is actually known as low gearing, a feature found on trucks and 4WD vehicles. It means, in simple terms, more traction at the wheels at the cost of speed. So, in high gearing you’ll do 20 MPH at 3000 RPM in second, and in low gearing, those parameters will only get you to 10 MPH or so. Which means more control in dirt, more pulling power for towing something heavy. These vehicles usually have an extra stick to select the gearing mode.
Low gears on an automatic are just that: the lower gears a stick car has as well. On flat non-icy roads with no tow load, they’re not really necessary. But as already explained by others, you might need them in other conditions. Also, bear in mind that an automatic car will understeer sooner (in case of a FWD) because the engine will be idling in the highest gear when you brake and steer into a corner (unless you’re driving something sophisticated like a BMW 540i Automatic - they sometimes are programmed to downshift automatically as well. Then again, the BMW is a RWD, but yuo get the idea ;)). You’d put a stick in second gear, and have the right amount of torque at the wheels while cornering, thus you’re in more control over possible understeer if and when it happens.
Darqangelle: constantly and routinely downshifting in an automatic is a BAD idea, unless you’re driving something like an Audi or Porsche with tiptronic (a dual-purpose automatic with the ability to shift as well), or a similar device from another brand (BMW has them as well). If you do it with a conventional automatic, you’ll wear your clutch our sooner than normal. And that’s the least of your worries.
Years ago I was with a friend who drove like this picking his car up from the shop that fixed his tranny. As he was paying the bill he asked if this was a common problem. With a completely straight face the guy behind the counter said “nah, just to ricky racers” sending streams of my soda out my nose.
I don’t understand the rational myself. Downshifting into the corner is a really bad idea as it results in uncontrolled braking of the back wheels which can be dangerous as hell. Once into the corner you want to accelerate to put more weight on the back tires and a downshift, in needed, will occur anyway when you hit the gas.
No, that was not my point. I meant that an automatic will have less available grip because it is idling in top gear. A stick, downshifted to second, will be in the right power band range to have control over potential understeer (assuming FWD here). Also, downshifting in a stick gives you clutch control, which means the rear wheels won’t just lock up.
That could happen to an automatic you slam into second right before a turn, and yes, that does pose an additional danger to the lower grip described above.
…heh, you’re assuming of course that I would own a car with an Automatic tranny.
Those were rentals.
…I’m a baaaad boy. 
In my region of the country (Appalachian mountains) we get a fair amount of snow, and Drive1 works wonderfully in this. I also know that while I am going down hills or steep inclines, I always use the gear right below Overdrive.
Only 3rd?
Depending on how quickly you’re going, I’d be more inclined (get it? “Inclined”? Huh? HUH?) to select second to ascent a steep slope of snow.
…how did I get so alliterate?
Minor correction: granny low is NOT the same as (regular) low gear. A granny gear is far lower than a normal first/low gear, e.g. 7.5:1 as opposed to 3:1.
ClogMod: Actually, she was talking about an extra-low 1st transmission gear some trucks have. I think you mean a 2-speed rear axle or 4wd transfer case, right?
As for the OP, the advantages are: more torque for towing; engine braking on steep hills; better acceleration for racing (the only auto I’ve found that upshifts at the “right” speeds is the Chrysler 727…hits 2nd gear at 40mph and 3rd at about 60mph at WOT :))
That’s not exactly what happens. Well, it does, but there is a certain lag time. Automatics shift based on RPM’s, not speed, traction, etc. So if you are going up a steep hill, especially in mountain areas, as soon as the engine hits a certain speed, it shifts up, which may not be optimum for the actual driving, and the car loses power to the point where it will drop a gear by itself. This can go back and forth and make for unpleasant driving as well as being a little rough on the tranny and the torque converter. For normal city driving, there’s no need to ever manually shift, but in some areas and conditions, it is a great benefit to be able to take high and even mid-gear out of the equation.
(Talking about “older” automatics. Many new ones on performance cars and the newer Cadillacs are different with all the technology in them.)
Going down steep hills when there’s ice on the ground is where I use the low gears. In particular, right near my place there’s a hill with a stop sign at the bottom. If you press on the brakes, you start to slide, and then how do you stop at the bottom? And if you don’t brake at all, you’re momentum takes you faster than you want, and you REALLY can’t stop at the bottom. So you put it in first gear, and the engine stops you from going too fast, and you can safely stop at the bottom (usually).
What about downshifting to pass someone? That would seem like a useful application.
My dad’s 1968 Oldsmobile Ninety-Eight automatically downshifts to second gear when you floor it. I have no idea if it is a common feature though.
If it is a common feature, disreguard this whole post as the car would do it automatically. However, if it is not, well, just be thankful that you can manually select 2nd gear.
It is a common feature, but many times it still takes the car a few precious seconds to figure out…
“Floor it?”… “Okay, I’ll slip into 3rd”…“not enough? Oh! You mean FLOOR IT! Ah well, second it is, then…what? You’re braking… but we could’ve passed the bus before that 18-tom semi would hit us.” … “Alright, fourth it is again. Geez make up your mind.”
…
I HATE automatic transmissions.
… not as much as those annoying “Shift now!” lights the big three feel they have to put in their standard-transmission cars,
…but that’s another thread.
I was specifically referring to the low/high gearing option that most Jeeps/4X4s have. A Suzuki Samurai will have a simple stick for it (much like a manual gear lever), and a fancy 4X4 like a Range Rover has an electronic switch for it (I think). In any case, it basically cuts the ratios of all gears in half. Or doubles them, whatever. You get the idea. 
The Granny Gear you speak of in trucks is unknown to me. I DO know some large trucks have “half gears”. When they’re driving empty, or just the tractor, they will use the standard gears (usually a 5 or 6 speed manual). However, when they’re fully loaded and going uphill, they have “in between gears” at their disposal. I think this usually works with a button on top of the gear lever. Basically, it turns a standard 5 speed into a 10 speed manual.
The automatic downshift feature on automatics that is referred to above is known as the “kick-down” feature.
I tried to stay away from this one as long as I could…but now I just can’t resist any longer.
I used to design and calibrate automatic transmissions for one of the Big 2 (Daimler Chrysler? No longer can refer to them as one of the Bigs).
Darqangelle I used to hate automatic cars too, until I worked in the field…and developed respect for them. I still prefer driving cars and trucks with M/T’s though.
Okay to answer the OP. The reason you can select 1st gear on the PRNDL selector is for those occasions where you might want to pull a load slow, or move your vehicle with high wheel torque through whatever situation you feel warrants it, w/o going to any higher gears…Descending a hill while offroading comes to my mind as an ideal situation where you would want to use the 1st gear selection. If you don’t ever need to do this, just leave it in Drive.
Side note:
I’ve always known “Granny Gear” as the term that is used for the lowest gear on some of the older model pick-up trucks and even some of the newer pick-up trucks with manual transmissions. When you don’t have a load on your truck, that lowest gear ratio is pretty much overkill. Most of the time you just start out in 2nd gear to avoid having to shift at 5mph at every stop light etc. When you are pulling a trailer or have your bed loaded down, then you might need the extra wheel torque to get going. I don’t know much about semi-tractors, but I know they have a multitude of gear ratios to choose from, and I’m sure they just use the gear that will keep them moving without sputtering when climbing hills…etc.
In an automatic transmission, starting up under load or w/o load does not matter…the torque converter takes care of variations in wheel torque requirements…in a way.
Then, based on throttle position and vehicle speed, the vehicle determines when it should shift 1-2, 2-3, 3-4, and if it’s a five speed auto, 4-5…and if conditions are right…the torque converter will lock together the input and output to form a “solid” connection between engine and tranny…on downshifts, it depends on how much you “step-in” to the throttle. If you go heavy on the throttle, like in a passing situation, you might downshift 4-2, or 3-1…skipping the gear in between. If you’re light on the throttle you might just get a 4-3 or just a torque converter unlock…Oh, and another thing, the only time an A/T shifts based on RPM is at wide open throttle…that’s to protect the engine from over revving.
We have tried to make A/T’s better for climbing hills so they will not shift cycle between a higher and lower gear every time a car deccelerates due to the inclination of the hill. The vehicle measures acceleration as well as speed and based on this, will determine whether or not it’s a good idea to upshift, even though throttle position and speed say it should. If it’s not accelerating enough, we assume that a downshift will need to occur soonafter, so no upshift is allowed until acceleration improves.
And yes, to save your brakes when decending mountain passes etc, A/T’s should be downshifted out of overdrive into plain old drive and sometimes down to 2 to employ the help of your engine to slow down the vehicle and stay under control. The “2” selection on most A/T’s will keep the car in 2 no matter what the conditions…even if you stop and start going again. Drive as opposed to Overdrive will use all the gears below the overdrive gear as if the overdrive gear does not exist. Other than that, the shifting occurs normally.
I hope this is not too jumbled up to make sense.
The “low range” gearing on Jeeps, Land Rovers etc, is due to the fact that the Transfer case has a gear reduction capability built into it. For those that don’t know, the transfer case engages or disengages the front axle driveshaft on 4WD vehicles. (those that are not full time 4WD) Normally, on most older 4x4’s the transfer case is disengaged to the front driveshaft and does not affect the gearing of the vehicle, when you engage the tran case, you have the option of engaging with or without reduction in gear ratio. I.e. 4 High or 4 Low
So, in addition to the gear your transmission is in, the output to the axles is reduced further if your transfer case is in High Range or Low Ranges.
I forgot to mention that the driveshaft speed is also reduced by a another ratio at the axle differentials before transferring the motion to the wheels.
Automatic shifting is not solely based on rpm’s. Mostly it’s determined by computer today. But even before computers crept into cars either vaccume and/or throtle position was a factor in gear selection.
Manually downshifting can be useful while going up/down hill, pulling a heavy load, when you need that little extra speed and the trany delay to downshift will be too long, driving off road at slow speeds when you want more control, when you are crusing at a speed that causes the transaxel to constantly shift between gears and of corse when the kickdown bracket breaks and your car won’t downshift on it’s own.