After the recent update which changed rolling resistance on dirt roads I did some speed tests between the Tron and MTB. Zwift released another update a few days ago which improved the speed of the MTB on dirt and I redid my tests. I do not have any numbers for before the changes to rolling resistance were implemented but I think we have all experienced the dramatic speed loss on dirt roads.
Course: 4km long on the Jungle Loop. Start at the first stone arch, down to the turnoff to Alpe de Zwift and back uphill again to the arch.
Rider: 183cm 80kg 201watts average for the course.
iOS version 1.0.40510 (update that added the steering MTB trial and first changed rolling resistance)
Tron time: 11 min 23 secs
MTB time: 11 min 44 secs
iOS version 1.0.41097 (update that made MTB faster)
Tron time: 11 min 24 secs
MTB time: 9 min 53 secs
Results: After the first update the MTB was slightly slower than the Tron bike but after 1.0.41097 the tron bike remains the same speed but the MTB is now nearly 2 minutes faster over the 4km long course and significantly faster than the tron bike.
Zwift Insider tests have also shown the MTB to be faster than the tron over the full Jungle Loop (13:47 vs 15:00).
Note that the Zwift Insider margin is less than the margin I had. Perhaps this is because I was at 200w against the 300w that Zwift Insider did. A slower speed means aero is less important and rolling resistance has a greater impact.