In previous Forza games, benchmark tool calculated car performance in the fly when simply browsing upgrades. Here, you need to apply part to see changes.
I believe this to be a bug (and if it’s by design, then I’ll put this into the Suggestions Hub 
).
Platform: PC/Steam.
Just noticed this as well. I just don’t understand the amount of issues we are experiencing for a “completed” triple A title. Very frustrating.
Please, for the love of god, fix this. Having to install and uninstall parts constantly while toggling back and forth through menus, to see how my stats change is frustrating.
@T10ManteoMax You can mark this issue as solved now, update 2.0 fixed it 
Marking this issue as Fixed with Update 2.0 on November 14th. If you experience any persistent issues related to this bug after installing the update please upvote a new topic in FM Report New Issues
This topic reached 8 votes.