It seems pretty the same to me except that Autos is more restrict. Subset sites just drain audience. They could be just one stronger site?
Motorcycles could be included?
Motorcycle would be well served as a distinct area.
Here is a graph of the relative populations you are hoping to engage in the US.
Autos is far too general/voluminous topic - motorcycle input will be a vast minority.
I have the same issue with a merge to Motor Vehicle Maintenance and Repair. What percentage of motorcycle enthusiasts would want to be in a M&R group? Also, the penalty for failing to properly repair a motorcycle means far less people will attempt two wheeled repair over car repair.
Traffic laws, training, safety and equipment have distinct effects on motorcycles that distance them from both bicycles and automobiles in significant ways.
The social aspects of motorcycle ownership are considerably different than automobile ownership in most western countries. I think the value of having a specific site would encourage motorcycle users from all continents to share how these machines affect their lives.
Motorcycle organizations in the real world have passionate, high percentage participation as compared to motorcyclists advocating within automobile organizations. I would expect SE to get more expert participation from a motorcycle area than an auto area. Also the pure numbers of auto users would dilute the impact of motorcycle participation.
If the goal is to have high quality participation of motorcycle owners, please keep things separate and let motorcycle discussion live or die based on participation and don't kill it by design.
Personally, I'd prefer one big site including all aspects of motorized vehicles, even though I am mostly interested in the Maintenance and Repair. The beauty of the tagged question is that a multitude of diverse questions can coexist on the same site. Also, the more varied the group of participants, the faster the site can get off the ground!
In that case, naming the site something like "Motor Vehicles Exchange", would be a better idea.
But this is just my opinion.
I think motorcycles is a distinct enough subset that it shouldn't be included.
Speaking for myself, motorcyclists tend to think of themselves a disparate group, and wouldn't "enjoy" being lumped in with Autos.
Though it's anecdotal, it's worth noting that on reddit, /r/Autos has 5,660 subscribers, while /r/motorcycles has 3,310.