There is a little bit of truth in just about everyone's posts above, but I will add a couple of things that may help clear the mud (or not?!).
First of all, you can get a speedometer calibration unit made by Cyberdyne and sold by Summit. It allows you to pace a car with the correct speedometer, and the set yours to the same reading. I think it is about $70. Some people around here have used them.
Regarding 3.73's, well there are good reasons why you wouldn't want them in an SC. For one thing, a stock SC peaks power at 4500 rpm. A highly modified one peaks at 5500. In either case, you will find yourself past your HP peak in 4th by the end of the 1/4. Not good for 1/4 times. Shifting into 5th is gonna cost you a good 10 rwhp due to internal transmission losses, again not beneficial. Say nothing of the extra time lost for shifting.
That being said, I have 3.55's in my 5spd. It runs very well at the track, and I don't have traction problems. It does get monotonous on the highway running 3000rpm for long trips - and yes, I've done cross country several times with them. They also cause the motor to run a significantly hotter than it used to.
Finally, there is a big difference in the power curve of a pressurized motor than an N/A one. Check what the turbocharged 5.0's are running for gears. Did I see 3.27's? Yup I did. See, we have a broad, flat torque curve and a HP curve that really doesn't curve much at all. Compare it to the curve on an N/A motor, especially a high HP one, and you will see that even for racing, a tall rear gear that forces the motor to rev to redline quickly is not helpful for us like it is for an N/A motor.