I got my car, assuming it had stock gears. After running out 4th gear and it stopping just short of 120mph, I did some research and it was looking like I had 3.73s, based on 4th gear mph/rpm and 6th gear mph/rpm (using a GPS). On my new tune, I listed 3.73s and my specific 18" tires for the speedometer to be calibrated correctly. I get the tune back, go to the race track. As I'm crossing the finish line, the speedometer reads 90mph, yet my actual trap speed is 105. I check back with the tuner, he looks over my tune, and it is in fact set up for 3.73s and 18's.

Any ideas? Like I said, everything pointed to 3.73s before, but now it's off?

 

What tires do you have on your 18" wheels?

 

Hankook ventus rs2 315/30/18

 

Not sure if I understand, but is the RPM vs. MPH inaccurate on the street?
You can easily check your gear ratio by jacking the rear up, chalking a line on the drive shaft and tire, spin the tire and see how many revolutions the drive shaft turns per one revolution of the tire. If you indeed have 3.73 gears then the DS will spin 3.73 times to one revolution of the tire.
It may be possible your tires are growing enough to change up the final ratio as well.