It would be interesting to pop into the debugger and see what’s going on but I haven’t built FiveM locally from source yet… maybe it’s time.
The unsupported type ? error message you’re getting for nInitialDriveGears may be a clue.
The other two fields really don’t appear to be supported, it looks like FiveM have only implemented a subset of the fields, you can find them in the HandlingLoader class, you may want to post a feature request to get them added.