Quote:
Originally Posted by ItsGary
55lbs is a lot. That's were your 1mpg increase/decrease comes from
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True, I think that's a factor, though I suspect not the major factor. The ZF8 is 20/31lb more than 6MT (on M240i/230i) but gets a +3MPG boost instead of -1MPG. The 24lb (<1%) weight difference between ZF8 and DCT7 is hard to explain a 4MPG (>10%) differential.
I think there's something else that is inefficient about the DCT vs. the ZF. Maybe the DCT's more aggressive shift programming or lack of a super tall 8th gear hurts it. I have driven a DCT M2, but not long enough and not in normal enough traffic to intuitively learn the programming.
To check this theory, I did the math for cruising RPM at 60MPH: ~1825rpm on the DCT for the M2 and 2300rpm on the 6MT (they have the same final drive ratio, but 6th gear on the manual is 0.85 and 7th on the DCT is 0.67). Very odd to me how cruising at 26% lower revs does not make up for the 55lb difference. If you think cruising at the higher revs is more efficient for the M engines, then the DCT should just downshift to 6th.
For the M240i, the cruising RPM at 60MPH is ~2130rpm (6MT) or 1475rpm (8AT), a 44% rev difference, which is good for a 10% highway MPG boost.