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      01-19-2012, 09:10 AM   #1
dead_billy
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Transmission ratios

Hi everyone! While waiting on my X1 sDrive18d I discovered this great forum with lot of intersting info. It seems that lot of people is following this forum and some with really impressive technical knowledge so I hope that someone can help me with following dilemma.
I was surprised when I discovered that there is significant difference in gearbox transmission ratios on s18d compared to s20d, x18d and x20d (all have same ratios)!? What does this difference means for agility and fuel economy?

Transmission ratios from official catalogue:

Gear - s18d - x18d/s20d/x20d
1(:1) - 4.00 - 5.14
2(:1) - 2.11 - 2.83
3(:1) - 1.38 - 1.80
4(:1) - 1.00 - 1.26
5(:1) - 0.78 - 1.00
6(:1) - 0.65 - 0.83
R(:1) - 3.19 - 4.64
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      01-20-2012, 06:42 AM   #2
jctg
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You do need the final ratios as well for a valid comparison.
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      01-20-2012, 07:59 AM   #3
dead_billy
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Final ratio s18d - 3.23
Final ratio x18d & x20d - 2.79
Final ratio s20d - 2.64

So identical setup for xDrive models (x18d and x20d). Why such big difference between s18d and s20d?
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      01-20-2012, 10:30 AM   #4
nockenpaul
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The s18d has longer overall gear ratios. So if you just compare the ratios, it results in better fuel economy but worse agility for the sDrive 18d.
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      01-21-2012, 08:53 AM   #5
jctg
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Quote:
Originally Posted by dead_billy View Post
Final ratio s18d - 3.23
Final ratio x18d & x20d - 2.79
Final ratio s20d - 2.64

So identical setup for xDrive models (x18d and x20d). Why such big difference between s18d and s20d?
By looking at just the gear ratios in your first post, the s18d is the odd one out.

Looking at the final ratio, the s18d is also the odd one out, with the final ratio for s20d and x18d and x20d being fairly similar (the s20d being longer as it is also lighter with no awd).

Once you mulitply the gear ratios by the final ratio for s18d and s20d, you get the following: (the first number being s18d and second s20d


1st gear 12.92 13.57
2nd gear 6.82 7.47
3rd gear 4.46 4.75
4th gear 3.23 3.33
5th gear 2.52 2.64
6th gear 2.10 2.19

The maximum difference is 10% which may be due to differences in the torque curves of the two tunings. Or that BMW decided that people going for the s18d want economy far more than performance anyway so they went for longer gearing. And/or that they have a certain headline MPG/KmL figure that they want to achieve.

As to why BMW chose to go long in the gearbox and short on the final ratio - I have no idea. Other than one would have its driveshaft spinning much faster. But the end result going out through the half-shafts are not "that" different.
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      02-09-2012, 02:28 AM   #6
Kampungboy
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How about DS mode? Does it still have 6 speed or less?
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