Trying to figure out what gear I have...

[xspeed02]

When I bought the car last year all my friends told me they thought my car had an aftermarket gear in it. I really didn't know what to think because it was my first stick. I was wondering if its possible to tell what gear is in the car by how many rpms you're at in 5th gear. I seem to remember a program out there that calculated that for you. But I figure since so many of us have different gears I can get a ballpark idea, and atleast know if the guy before me put a gear into it. At 65 miles per hour I'm just above 1500 so I'd say roughly 1700rpms in 5th gear. The guy before me said he never put a gear in the car and he was the only owner. But I dunno, it just seems to run too strong. Its a 90GT 5spd (obviously) with 150k miles on it. Thanks guys.