Always 4-speed autos?

[WRC9000]

Anyone else ever wonder why almost all automatic trasnmissions come with only 4 gears? Ussually if you get a stick, you'll have five or six, but just about every auto is always only four, with the exception of a few trannys that allow you to use a push-button/paddle shifter, which come with 5 or six too.

My mom's lexus is auto, but with the option to use shift buttons on the steering wheel, and it comes with five.

So why in all-auto trannys do they only have four gears? Is it just becuse people who opt for manual/semi-auto just want to have more choice when selecting gears and actually care about the gears, and that people who get full auto don't care, so thye just use the least they can get away with?

Do four speed transmissions have longer gear ratios? From a purley functional standpoint, are trannys with more gears better with acceleration, becuase each gear is a bit shorter and offers more torque?

[MagicRat]

An automatic transmission can get a wider useful range from each gear because of the torque converter.

This device allows for 'slippage' at lower engine revs, and as the engine revs increase, this rate of slippage reduces, until at high revs there is almost none (and when the converter locks up at highway speed, there is no slip at all)

This allows an automatic to have the same range of function with fewer gears than an manual. 4 speed automatics do also have wider gear spacing than a typical 5 or 6 speed manual.
Some older overdrive 4 speed manuals (Ford's SROD trans) and many old 3 speed manual transmissions had similar spacing to the gears in an auto trans.

Finally, not all automatics are 4 spped transmissions. Many older ones are 3 speeds, the Chevy Powerglide trans used only 2 gears. The GM Turboglide trans, made for one year only (1958) only had ONE gear, and used torque converter slippage exlusively to moderate the effective gear ratio.

[curtis73]

Most new GMs get a 6 speed auto. MagicRat hit the nail on the head. I'll add that a 4-speed auto is much cheaper, too.

[Moppie]

Originally Posted by WRC9000

Anyone else ever wonder why almost all automatic trasnmissions come with only 4 gears? Ussually if you get a stick, you'll have five or six, but just about every auto is always only four, with the exception of a few trannys that allow you to use a push-button/paddle shifter, which come with 5 or six too.

My mom's lexus is auto, but with the option to use shift buttons on the steering wheel, and it comes with five.

So why in all-auto trannys do they only have four gears?

First of all you have just contradicted yourself there.


And to add what the Magic Rat said, its a matter of complexity and need.
During the 60s no one knew how to make a 4 speed auto that was going to be affordable, so they were all 3 speed (some 2 speeds). Then as the technology improved so did the number of gears availiable. 4 speeds became popular in the 80s, and 5 speeds started to show up in end of the 90s, and are not becoming more and more common.
Borg Warner now make a 7 speed auto, that is used in a variety of BMW's.

[KiwiBacon]

Originally Posted by MagicRat

An automatic transmission can get a wider useful range from each gear because of the torque converter.

This device allows for 'slippage' at lower engine revs, and as the engine revs increase, this rate of slippage reduces, until at high revs there is almost none (and when the converter locks up at highway speed, there is no slip at all)

This allows an automatic to have the same range of function with fewer gears than an manual. 4 speed automatics do also have wider gear spacing than a typical 5 or 6 speed manual.

And while the torque converters work excellently. Their efficiency does suck.

I've wired in a lockup switch for the 4sp auto torque converter in my daily driver. It gives many of the benefits of a manual, but it also clearly shows that four gears are not enough with a locked up TC.

More gears combined with a lockup TC will have a positive effect on fuel economy. Especially in situations involving a lot of speed changes (i.e. city driving).