How do I ID a 4 speed, overdrive or not?

Thanks for the info. I went under today and its got one lever up and one down. But a major tip off is that its all aluminum, seems like its an overdrive. On the drivers side I saw the #'s C13546 B , any ideas what its from? I quess thats why I can cruise at 70 and run 2000RPM with 2.91's.

So Next questions is, with an overdrive tranny what kind of gears can I run with a 27-28" tire and still be streetable?

Thanks

Butch