i think it should be only a couple miles dropped but not much. most people recommend 4.10's for automatics and 3.73's for sticks. here is something else i read off a website:
What rear differential gear do you recommend?
2V GT: The GT reacts to gear changes much the same way a 5.0 does although the real problem is that they have less torque than a 302.
For stick cars we recommend 3.73 gears. Because of the lack of low end torque, we don't feel 3.55's are enough. The 3.73 gives an excellent increase in acceleration and top speed isn't decreased because you are able to wind out overdrive further. With a 3.73 your rpm in 5th gear at 60 mph will only be 2005, not at all what you might call "too high". As we said, your top speed will not decrease when putting lower (numerically higher) gears in a car with overdrive. You will simply wind out 5th gear further than you could with the higher gears (numerically lower).
For AODE cars, 3.73's are still a good choice but we recommend 4.10 gears. A 4.10 may sound extremely steep, but because the AODE's 1st gear is higher (numerically low) and 4th gear (overdrive) is steeper than the T-5, it's a very good choice for the driver interested in making their car perform better.
4V Cobra: The Cobra has an entirely different engine than most people are used to. It performs best at high rpms, upwards of 7000. It's power band is relatively high due to the deep breathing nature of the two intake valves. When gearing a Cobra, 3.73's just aren't enough. They don't allow the engine to reach its powerband quickly enough. 4.10 gears are an excellent choice and 4.30's are even better. This may sound very steep if you're used to the average 5.0 which needs to be shifted by 5600 rpm. But since this engine will easily rev to 7000 it needs the extra gear and it likes it. 4.30's are our #1 choice, naturally aspirated or supercharged. At 80 mph in fifth with 4.10's the engine only turns about 2700 rpm, completely tolerable.
