So, let's change from this:
Bicycles is a question and answer site for people who build and repair bicycles, train bicycling, or commute on bicycles. It's built and run by you as part of the Stack Exchange network of Q&A sites. With your help, we're working together to build a library of detailed answers to every question about bicycles and bicycling.
Neither tag has a description, so it's not surprising they get used inaccurately.
I would have thought they were different, but if we cannot define them to be substantially different, then yes, they should be merged.
Suggested definitions (improvements invited)
Diet definitions applicable to cycling would be
the kind and amount of food prescribed for a ...
The tag should only be used for that first case. The second ("the question is about either A or B, but not both") is not how to properly use tags.
I reviewed site usage of that tag. The questions were all either about upgrading an existing bike, or about shopping for a new bike. I cleaned the tags up on all those questions. I also made "new-bike" a synonym ...