True, however so will bigger different size wheels and tyres. I'm looking for a little more off the line and more importantly a limited slip. If I had a 3.5FD and VLSD I would be content with that too.

 

Instead of trying to figure out what you have (which seems to be running everyone in circles), just get a quaiffe helical LSD with 3.9 gears.

3.9 is the best performance/drivability ratio.
You can do 4.01, but your rpms will be at 4k at 80mph, so that's a whole 1krpm more than normal. This is based on stock tire diameter tho. If you have an overall smaller (than stock)tire diameter it'll be even worse, and if you have a larger diameter, you'll fare better with the more stout gearing by basically correcting your final drive a bit.

It's important to choose your wheel diameter before your differential gearing.

 

True, but irrelevant. I mentioned the speedometer because of what I said in the previous post regarding different threads on here.

Are you sure its not a VLSD? Does you car have the sports package?

 

Best way to tell is to open the diff cover and read the ratio directly off the ring gear. Something like 48:13 (3.69).

Another way is to tape a long ribbon to the driveshaft, spin the axles, and count how many times the driveshaft spun around with 1 rotation of the axles.

Nice seeing my old posts being quoted, lol