Taking the transmission and differential gearings as posted above at face value, I calculated the following revs at the wheel vs revs at the engine using engine rpm divided by transmission gear ratio divided by differential gear ratio. Unless I got the calculation wrong (do not think so and my xls skills are good) and/or lower revs at the wheel for certain engine rpm is not helpful (I am sure it is helpful as Power and Torque at the wheels will be higher at lower rpm), the 8 speed with 3.31 differential is ahead of the 6 speed except for perhaps one more gear change to get to say 60 or 75mph. Bottomline there is little difference, eventhough the 8 speed should be faster, since a) the engine is the same and b) the weight and c) the aerodynamics (for as aero our 200's are...), so that is why it is very close in 0-60 and fuel economy.
I am sure I could have laid this out clearer as well, however enough effort on this. All in all the gearing on the 8 speed is more favorable then the 6 speed including less need to re gear when you go up in tire diameter. The 6 speed is still a great box as it has proven itself already.
rpm | 6 speed first gear | 8 speed first gear |
1000 | 76.80 revs at the wheel | 63.07 revs at the wheel |
3000 | 230.41 revs at the wheel | 189.22 revs at the wheel |
5500 | 422.42 revs at the wheel | 346.90 revs at the wheel |
rpm | 6 speed second gear | 8 speed second gear |
1000 | 130.49 revs at the wheel | 107.51 revs at the wheel |
3000 | 391.46 revs at the wheel | 322.54 revs at the wheel |
5500 | 717.68 revs at the wheel | 591.33 revs at the wheel |
rpm | 6 speed third gear | 8 speed third gear |
1000 | 255.75 revs at the wheel | 164.19 revs at the wheel |
3000 | 767.26 revs at the wheel | 492.58 revs at the wheel |
5500 | 1406.65 revs at the wheel | 903.06 revs at the wheel |