viewport configuration -> statistics, tick triangle count and selection and you should be able to see more detailed info, but you probably already know that!. My suggestion for the tires if you want to cut them down is to make the treads floating geometry, and get rid of some of the rim edging. I would also NEVER use optimize on a game asset, it just ruins the model. I might see if I can do a quick paintover of the things I might look at to lower the polycount.
-edit-
Very Similar suggestions to rawmeat overall, but anywho, did before I saw him post! Basically where possibly concentrate only on creating a good silhouette and and make use of floating geometry to avoid breaking up faces too much. Realistically in the Source engine you would probably do this with brushes for the most part I think though.
-double edit-
I would also suggest using splines plus loft and a ngon when creating rails, that way you can build all your rails an easily change the number of sides they have depending on how your poly count is going. Also, I highly recommend being as stingy with polies as possible right from the start and adding details later only if you have spare.