Why usually most automatic cars are tall-geared?

[culver]

I don't mean any gear alone, i mean the OVERALL RATIO.
Take for example Peugeot 206 1.6 16V (TU5) with AL4 transmission with the following ratios:

2.725
1.499
1.000
0.711
Final drive ratio = 3.65

And here are the overall ratios:
9.94
5.47
3.65
2.59

If you look at the overall ratios, you will see that they are insanely tall for a tiny, torque-less engine like TU5 which has only 147nm@4000RPM

Now look at the following ratios for the Mercedes 190E W201 with 4-speed manual transmission:
3.91
2.32
1.42
1.00
Final drive ratio = 3.23

Overall ratios:
12.62
7.49
4.58
3.23

190E has much better torque curve in low revs and it has 178nm@3500RPM but the ratios are much better than the insanely tall ratios of 206 which has a tiny, torque-less engine which peaks at 4000RPM.

Comparison of overall ratios:

190E 206
12.62 9.94
7.49 5.47
4.58 3.65
3.23 2.59

My question is that why the ratios of 206 should be much taller than 190E ratios despite the fact that 206 needs much shorter gearing to compensate for it's lack of torque at low revs?

You missed something I said before. The torque converter also offers a "gear ratio". According to wiki this torque multiplication ranges from about 1.8:1 to 2.2:1 [ame="http://en.wikipedia.org/wiki/Torque_converter"]Torque converter - Wikipedia, the free encyclopedia[/ame]
So take your automatic ratios and multiply at least first gear by about 2:1. That makes your first gear range as low as ~20:1. Now you only get that 20:1 when the car isn't moving. As the TC speeds up that ~2:1 becomes 1:1. So the TC has a much lower "first gear ratio" when you look at how many times the engine torque is multiplied. I use quotes because unlike the manual where both torque and speed are multiplied, the TC only multiplies torque, not speed.

Now in my example I only talked about first gear just as the car gets rolling. With older 3 speed and even 4 speed autos (or even the VERY old 2 speeds) you are often running the TC in some range where it isn't delivering 1:1 input to output speed and toque. In other words, it isn't locked up. Thus when going up a hill in top gear your net ratio isn't just the gear ratio * final drive you listed. It also might included a 1.2:1 torque multiplication in the TC.

When GM first started experimenting with automatic transmissions they initially had a 3 speed with a hydraulic coupling that offered no ability to multiply torque. It was effectively a viscous coupling. That transmission was a 3 speed design. When they first started using a TC they had only two forward speeds. Still, with only two speeds that TC equipped auto had a greater total torque multiplication range than the 3 speed auto. This is why for a long time a 4 speed auto could tow just as well as a 5 spd manual and wouldn't rev any higher on the freeway. The TC effectively gave the car an extra gear ratio at least when it came to how much torque (not speed) the engine could apply to the ground. Basically, the TC allows a gearbox with a narrower total ratio range to behave like it has a larger range.

[karabiner98k]

You missed something I said before. The torque converter also offers a "gear ratio". According to wiki this torque multiplication ranges from about 1.8:1 to 2.2:1 Torque converter - Wikipedia, the free encyclopedia
So take your automatic ratios and multiply at least first gear by about 2:1. That makes your first gear range as low as ~20:1. Now you only get that 20:1 when the car isn't moving. As the TC speeds up that ~2:1 becomes 1:1. So the TC has a much lower "first gear ratio" when you look at how many times the engine torque is multiplied. I use quotes because unlike the manual where both torque and speed are multiplied, the TC only multiplies torque, not speed.

Now in my example I only talked about first gear just as the car gets rolling. With older 3 speed and even 4 speed autos (or even the VERY old 2 speeds) you are often running the TC in some range where it isn't delivering 1:1 input to output speed and toque. In other words, it isn't locked up. Thus when going up a hill in top gear your net ratio isn't just the gear ratio * final drive you listed. It also might included a 1.2:1 torque multiplication in the TC.

When GM first started experimenting with automatic transmissions they initially had a 3 speed with a hydraulic coupling that offered no ability to multiply torque. It was effectively a viscous coupling. That transmission was a 3 speed design. When they first started using a TC they had only two forward speeds. Still, with only two speeds that TC equipped auto had a greater total torque multiplication range than the 3 speed auto. This is why for a long time a 4 speed auto could tow just as well as a 5 spd manual and wouldn't rev any higher on the freeway. The TC effectively gave the car an extra gear ratio at least when it came to how much torque (not speed) the engine could apply to the ground. Basically, the TC allows a gearbox with a narrower total ratio range to behave like it has a larger range.

Thanks culver, you are really a professor!
So, according to what you say in first gear for example 190E may have a total multiplication as high as 20:1

How is it possible that TC multiplies torque without changing the engine's speed? (When the ratio becomes shorter, the RPM should go higher)

If TC multiplies torque, then more torque should reach the wheels. But when magazines test automatic cars in manual mode and measure in-gear times (60-100km/h in 4th 80-120km/h in 5th & 6th), the results are always very slow relative to engine torque and the weight of the car.

For example:
Mazda 6 (5-speed automatic) with 1320kg
2.0 16V 147hp@6500 184nm@4000

80-120km/h in 5th gear = 29.6s !!! (extremely slow)

5th gear ratio = 0.69 Final drive ratio = 3.86

Toyota Corolla (5-speed manual) with 1245kg
1.6 16V 124hp@6000 157nm@5200

80-120km/h in 5th gear = 17.8s (much much better than Mazda 6)

5th gear ratio = 0.82 Final drive ratio = 4.31

I don't think this huge difference in times is related to 75kg difference in weights.

You said that gear ratio * final drive ratio is not always the case and TC also can multiply torque.
But why Mazda 6 is still very much slower than corolla despite having more engine torque in 1200RPM lower and that TC multiplication effect?

[karabiner98k]

Brother had the Cosworth 190E and it was a LOT lardier than that.
200kgs more than the 206 you've cited.
AND if you take the even lardier "Normal" variants with auto then I think you're up into 1700kgs

I doubt that even the six cylinder auto 190Es were up to 1700kg. 1500kg at most I would say.

That 190E which i mentioned weighs between 1100-1140kg.
Here is the source:
W201-190E-Abmessung – Mercedes-Benz Classic Wiki - Online-Lexikon rund um Mercedes-Benz Oldtimer

190E Cosworth has various versions. The heaviest of them is 190E 2.5-16 Evo II with 1340kg
Here is the source:
W201-190E2516EVO2-Abmessung – Mercedes-Benz Classic Wiki - Online-Lexikon rund um Mercedes-Benz Oldtimer

[Matra et Alpine]

yeah it was the full on Cossie version - awesome beast
But as clarified, my high end weight confused the laden My bad

As Culver has said I'd forgotten that TC multiplies by a factor dependant on design.
Any torque multiplication is determined by the engine speed and the difference in the input and output speeds. SO a "simple" multiplication of peak engine torque doesn't work due to the dynamic nature of the fluids moving in the converter.

Always reckon here is a good place to start to understand the mechanics of cars ... HowStuffWorks "How Torque Converters Work"

[culver]

Exactly how the TC goes about multiplying toque I don't really know. It's all in the fluid dynamic interplay of the three internal parts. Matra, rightly pointed out that the ~2:1 torque multiplication only occurs when the torque converter is "stalled". That condition only occurs when there is a sufficient difference between the input and output speed of the converter (probably around 2000 RPM but that would be specific to the TC). So in first gear when you hit the gas but the car hasn't started moving you are likely getting that ~2:1 torque multiplication. At almost all times as you are driving that ratio is probably around 1:1 though when under load it might be more like 1.2:1 (that is a guessed value). If you have a lock up TC then it will be 1:1. So when it seems like the engine revs up but the road speed doesn't change (but the auto hasn't shifted) what you are seeing is the input of the TC multiplying the engine torque.

Now the problem with the TC is it's not a gear ratio so you don't have 2x the torque thus 1/2 the velocity. When you are brake torquing (holding the brakes but pressing the gas) you might have 2x the torque and 0x the velocity. All that HP is being burned up as heat in the TC (thus the reason for needing a transmission cooler with some autos). The fluid flowing around creates the torque multiplying effect but it also allows for slippage and losses.

[darrylsaladino]

I don't think that GM's six speed will be that much much expensive to repair and with more gears the transmission fluid may stay cooler if the torque converter does less work. I think that to get six speeds there are probably 3 planetary gear sets.

[Rizaven]

Fuel economy is the obvious answer. Especially with older cars, having fewer gears mean that at stop and go traffic you'll be stepping on the accelerator harder to get off the line you'll be using more fuel. So the car compensate with highway mileage. Having two very low gear and a very high top gear means you'll have to rev higher on second gear before the transmission shift to third. When car companies started adding a fourth gear it was usually an Overdrive gear and was better for Highway driving but did not solve city driving. Constant Velocity Transmission was suppose to solve the city driving solution.