My Birkin with 205/50-15 tires, 3.9 rear end, and .84 T9 5th gear is turning 2875 rpm at 60 mph.
Here's a couple good gearing calculators, one from member JohnCh.
[/url]http://www.throttle-steer.com/downloads.html
http://www.our-nest.co.uk/Caterham/html_files/Files.htm