Hmmm. There is always some tach error, but it sounds like you have 3.73s. 3.27s were the only gears that came stock. With 3.27s you should be between 1800-1900 RPMs at 60 mph. If you are exactly at 2100 rpms @ 60 then you would have 3.70s (3.73 is the closest available ratio for the 8.8). If they were swaped by the previous owner then the tag will be inaccuriate. Maybe he meant 3.73 instead of 3.23?
The best way to tell is to jack up the rear two wheels up with the car in neutral. Put a chalk mark on the very bottem of the drive shaft and have a friend rotate the rear wheel exactly once, slowly. Count how many times the drive-shaft rotates for each rotation of the wheel. Conveniently if you have 3.27s the mark will end up on the driver's side. If you have 3.73s, it will end up on the passenger's side. If you have 3.55. it will end up pretty much right on top.
3.73s aren't bad for a Cobra. You shouldn't see any loss in milage, and your car should run a bit quicker. So unless you want to keep the car completely stock, or are looking to do some serious modifications, they will work just fine.
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