I sent Fidanza (aluminum flywheel manufacturer) a lengthy email, this is the response:
Good afternoon,
We unfortunately do not have any hard data tests showing results for gas mileage. Most people who purchase an aluminum flywheel are doing so for the performance aspect of it and do not even consider this portion before making their purchase.
I do apologize that we cannot give you a more definitive answer on this subject and that I could not find any prudent information while researching the subject for you. As you stated in your message I have found a lot of conflicting information as well as a lot of false information mixed in. From my personal knowledge of vehicle operations knowing that the flywheel will reduce the overall rotating mass on the drivetrain it will require less force to put the vehicle in motion but at the same time it will also allow the momentum of the vehicle to slow quicker as well. So it would save on the amount of gas required to begin moving it, but will require a bit more to keep it in motion, this alone may cause any decreased amount of fuel usage to be burned up. On a vehicle that is being driven in more stop and go type situations this may have enough offset to provide an increase overall MPG numbers, while the same vehicle being driven more on the highway may not see any increase and it may possibly see decreases. There are so many outside variables that can affect the mpg, most of these being uncontrollable by the driver it is very hard to say if any particular modification would cause an increase or decrease without being tested in a controlled environment.
From my personnel experiences with usage on my own vehicles, I saw a little gain on a daily driven Honda vehicle but I was not in any way shape or form trying to increase it. I would have days where I would do my best to keep a "light pedal" to help restore mileage but I also had days that it would be more "spirited driving" even with this type of driving i saw an overall increase of about 3-4 MPG on a total tank.
Dan Jenkins
Fidanza Performance
Technical and Product support
tech@fidanza.com