Why usually most automatic cars are tall-geared?

[karabiner98k]

Why usually most automatic cars have taller gearing than the manual cars?
This is particularly the case with older automatic cars.

[henk4]

Why usually most automatic cars have taller gearing than the manual cars?
This is particularly the case with older automatic cars.

to compensate for the mechanical and frictional losses, which result in higher fuel consumption?

[karabiner98k]

to compensate for the mechanical and frictional losses, which result in higher fuel consumption?

Thanks. Any other ideas?

[pimento]

Also with older cars there's usually fewer gears.

[IBrake4Rainbows]

Fewer gears means you have to use the available ratios more broadly.

[karabiner98k]

Fewer gears means you have to use the available ratios more broadly.

Yes, that's true but i have seen many cases in which the ratios of the automatic are still taller even if the number of gears is the same.
I mean overall ratios (gear ratio x final drive ratio) of the automatic transmissions are most of the time taller than manual transmissions.

[culver]

What do you mean by taller geared? Do you mean top gear or first gear.

Traditionally autos have a taller first gear because the torque converter increases the off idle torque of the motor. So instead of needing a low first gear to get going from a stop, the TC multiplies the limited off idle torque of the motor such that the engine doesn't stall with a taller first gear ratio.

As for the top gear, I've only noticed taller top gears on recent 6 speed autos. In that case they are doing a double overdrive like the Corvette has done for years. On the Corvette, people moaned that the tall top gear made the top gear acceleration really slow and thus even with 400hp a C6 required a downshift to get the same passing power as a V6 Accord. Well with an automatic the downshift is easy. The car does it for you so why not have a really tall top gear for good mileage and then a somewhat lower gear that is still tall enough that you don't notice your Civic just downshifted to go up a hill but low enough so that the Civic can make it up the hill with some feeling of power. Since the autos shift for you why not have the really tall top gear.

BTW, I'm not sure what the mechanical losses are in a modern auto but thanks to lock up torque converters I don't think they are much if any worse than manuals as you are moving. They are worse at traffic lights as the engine is under load vs being truly unloaded with a manual or DSG box.

[karabiner98k]

What do you mean by taller geared? Do you mean top gear or first gear.

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?

[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?

[Matra et Alpine]

A 206 is a LOT less weight than a 190E and so doesn't need as much torque and they'll meet better fuel consumption tests.

[karabiner98k]

A 206 is a LOT less weight than a 190E and so doesn't need as much torque and they'll meet better fuel consumption tests.

The truth is that 190E weighs between 1100-1140kg from 1982-1988 and 206 XT 1.6 16V Automatic weighs 1119kg.
So, they are almost the same in terms of weight.

[Matra et Alpine]

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
If you're comparing older/lighter 190s then my bad BUT then what is the point ???

[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

[Ferrer]

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