Q: What do the numbers and letters mean on an automatic transmission: 1, 2, D, L, S, etc?
A: That’s what’s known in the automotive world as the “prindle,” the pronunciation that engineers bestowed on the transmission gear selector because it commonly contained the letters PRNDL for Park, Reverse, Neutral, Drive and Low.
Drive, of course, covers all the forward gears — and because an automatic transmission selects those gears automatically based on throttle position, vehicle speed and other factors, Drive is the only transmission position most people select for forward progress.
Most automatic transmissions also allow you to select one or more positions past the PRND options. In some cases, what you're selecting is essentially a mode, like Drive, that allows the automatic transmission to do its own thing, shifting from one gear to the next, but within certain parameters: A Low (L) setting typically holds the transmission in lower gears than Drive for given speeds — as you'd want for providing engine braking to descend steep hills — but nowadays, it doesn't limit the transmission to one gear.
There's also the mention of S, which could be a Sport setting in which the transmission behaves similarly, downshifting sooner when you open the throttle to pass and holding lower gears longer for sustained acceleration.
Modern cars typically have just an L or S at the end of their PRND, but there was a time when automatic cars and trucks commonly followed Drive with 321 or even 4321. Though there have been exceptions, the numbers rarely meant the automatic gearbox would serve as a manual whereby the driver could shift into, say, 2nd gear and the transmission would refuse to shift up or down. Rather, each number simply represented the highest gear to which the transmission would shift automatically with the lever in that position.
For example, in a four-speed, selecting 3 would allow the automatic transmission to shift among 1st gear, 2nd gear and 3rd gear, but 4th gear would be locked out, accessible only by selecting Drive. What for? When driving in the mountains, a lower gear provides more engine braking and keeps the transmission from shifting as often, which can overheat it. Only a 1 position, if provided, would truly hold this type of automatic in a single gear ratio.
In some vehicles, switching the lever from D to the highest number means locking out the two highest gears, not one: Some automatic cars have a push-button on their gear levers or dashboards marked "O/D off" that locks out the overdrive gear, which historically has been the single topmost gear. Such a transmission marked PRND32 would actually be a five-speed, for which pushing the O/D off button limits the gearbox to 1st through 4th gears, and 3 tops it off at 3rd gear.
Only in recent years have automatic transmissions with manual modes proliferated, usually identifiable by the plus and minus positions on a separate gate alongside the gear stick, and/or by the presence of shift paddles or buttons on the steering wheel. On these cars, the manual mode usually means only the driver can shift, but in some cases they just lock out higher gears like the number position did in our old friend PRND321.
See the vehicle’s owner’s manual for more info. You might be enlightened about other ways to maximize performance or reduce stress on the transmission as well as by the wide variety of driving modes modern vehicles offer, typically through dedicated buttons rather than the stick.
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