[JimD]

The only logical explanation is gearing unless you want to theorize that the DCT clutch slips all the time (which is almost certainly not true because it would fail quickly if it did). Modern torque converter automatics lock up in top gear. So they do not have losses in top gear like the old non-lock up conventional automatics did. This means that the steptronic is as efficient in delivering power once in top gear as a manual or DCT transmission. But engines nearly always get better mileage turning slower. There is friction in the engine that is a function of how fast it turns. So if you spin the engine faster in top gear, you will get worse gas mileage.

It is not uncommon to have an automatic rated higher mileage than a manual. If you check out the final drive ratios you will find the automatic is spinning slower in top gear than the manual. My theory is that car designers do this because they are afraid manual drivers, like me, will complain if they have to downshift to pass somebody quickly. The automatic downshifts itself.

BMW has just set up the DCT for quicker acceleration than the steptronic. They may also worry that drivers will leave it in manual mode and complain about the acceleration in top gear.

If you eliminate gearing, a manual transmission will always get better mileage than a torque converter automatic. The torque converter has looses which are shed in the cooler ahead of the radiator. A manual, or a DCT, does not have these looses. But gearing is a bigger factor, especially in the highway rating.

A DCT will have some loss for the hydraulic system that shifts. But that should be pretty minor. If there is a cooler then I am wrong and it is significant. If there is no cooler on the DCT then the loss is not large enough to affect mileage more than something on the order of a tenth of a mile per gallon.

If it matters, yes, I am a mechanical engineer by education.

Jim