Why do auto trans. have fewer gears than manual?

Why do automatic transmissions usually have one less gear than their manual counterparts? For instance, my auto is a 4 speed whereas manuals have 5 gears. Why?

~THT

automatics are far more complcated than manual transmissions and therefore cost more. You can get auto trans with 5 and 6 sp gears, but they are just really expensive.

Space is often the issue, in many cars an automatic gearbox with more gears won't simply fit. In some cars they do but they are there limited by the increased cost but you can find them in for example Mercedes (I think they have up to 7-speed gearboxes).

One of the reasons is because automatic transmissions have torque converters which essentially allow a wider speed band for the torque available(called "slip").

With a manual transmission you have to gear so low to get a decent starting torque that you run out of RPMs requiring you to shift to the next higher gear. With an automatic, you can generate more torque at a higher "slip" RPM so you don't need such a low gear -- the engine RPM is higher, generating more torque because the connection is thru the torque converter so the output RPM is lower than the engine RPM but the torque is being transmitted.

Craig FL got it, good answer.

One of the reasons is because automatic transmissions have torque converters which essentially allow a wider speed band for the torque available(called "slip").

With a manual transmission you have to gear so low to get a decent starting torque that you run out of RPMs requiring you to shift to the next higher gear. With an automatic, you can generate more torque at a higher "slip" RPM so you don't need such a low gear -- the engine RPM is higher, generating more torque because the connection is thru the torque converter so the output RPM is lower than the engine RPM but the torque is being transmitted.

The torque converter is a great device, for this reason. The converter also produces lots of slippage at low revs, but as the engine spins faster, the amount of slippage is reduced, allowing the rpm of the trans to almost match those of the engine.

Back in the '50's, GM had two automatic transmissions.
The Powerglide had a torque converter and two speeds. The coverter slip provided a wide range of operation in each gear, so one only needed 2 gears.

The Hydro Matic was a 4 speed automatic. Yes, back in the '50's GM made millions of 4 speed autos, (and we all thought the 4 spd auto was first introduced in the '80's.
This trans had a 'fluid coupling' between the engine and transmission, which eliminated the need for a clutch, but but did not produce the slippage and torque multiplication of a converter. It needed 4 gears just to equal the range of the 2 spd Powerglide.

In 1958, GM made the 'Turbo Glide' trans which had ONE gear. It had a converter which had a variable slippage device built in, to allow a very wide range of engine rpm. In reality, these were somewhat sluggish, a bit like driving your car stuck in 3rd gear, all the time.

The torque converter is the reason of that automatic transmission cars have a higher fuel consumption than manual cars and all good new automatic transmissions can lock the engine output and transmission input axle so it isn't used when not needed. Of this reason the slipperage should also be minimized.

As for the numbers of gears the reasons are basicly cost and space, especially in FWD cars where space is a large issue. Otherwise a automatic transmission car would also benefit from the extra gear.

The other option is to use a CVT (Continuously Variable Transmission) which tend to use two cones instead of planetary gears. This permits the use of infinitely variable (between the limits of the gearing range) transmission ratios - which means the engine can sit in an ideal operating speed for economy, torque or power.


However, these have been limited in the past to smaller displacement engines although they are beginning to appear on 2 litre engines in Nissans. They offer some packaging advantages particularly for FWD cars and would appear to make some sense with hybrids (as it enables the electric motor to spin at a certain speed or make the most of the instant torque - generally electric motors have the ultimate flat torque curve except for maybe steam engines) and it reduces the need for slippage which is effectively allowing power to escape as frictional losses.