I'm very interested to follow this thread because I just had an argument/discussion about gears over thanksgiving. Any info here is much appreciated especially if any one wants to simplify the reasons for different gears and what the different sizes do (larger vs smaller)
Zero... However, the feel will be different. Changing from a 3.06 to 3.90 is called installing a poor man's supercharger on the Viper forum. The car will feel stronger, but you will be shifting quick and your top end will be much lower.
You should be aware that although a 3.90 rear will allow your engine to operate at a higher rpm and may give you a few tenths in the quarter, it will slightly decrease your horsepower and torque on a dyno and you will lose mileage (because you will be burning more fuel at the same highway speed because you are running at a higher rpm).
My personal theory is that there is an increase in loss factor that corresponds to an increase in axle ratio. The more "conversion" there is, the more frictional loss there is for that conversion. That's certainly not to say that taller axle makes a faster car, though. Naturally, the whole point of gearing and acceleration time is to squeeze as many cycles of engine top end into a target window...that will ensure highest avg hp applied. However, on the extreme end of short gearing, you still face mounting frictional losses from "conversion" and inertial losses from repetitively accelerating the engine rpm (if a manual gearbox, then the additive lag time for each shift becomes a loss, too). So somewhere in the middle is the optimal point where the combination of low frictional loss and avg hp yields the best time.