Quote:
Originally Posted by dead_billy
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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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.