^good info in this post^People who are sitting their cars up for the Eighth mile track sometime go to 4.30s. I've see people comment on 4.56s but I don't recall any forum member actually running them. I have know guys with track only cars with them.
Here is the problem with an auto. It only has three drive gears and an overdrive (a gear with a gear ratio higher than 1.00:1). First has a high 2.84:1 gear ratio. For comparison manual transmissions have a 3.38:1 first. Second is a fairly high 1.55 compared to a manual transmissions 2.00. To get that jackrabbit start that most people want it is generally thought that 4.10s are needed for an automatic.
But there is a price to pay. An auto also has the lowest overdrive gear of any of our transmissions. It's 0.70:1. A 2002-2004 manual transmission's overdrive is a higher 0.62:1. That means that an auto's highway rpm are going to be a lot higher. At 70 mph with 4.10 gears and standard size tires an auto will be spinning the motor around 2700 rpm. A manual transmission will be spinning it around 2400 rpm.
The reason the Eighth mile guys can get away with lower gears is because they don't run out of gear on the short track. On a quarter mile track you have to worry about running out of gear in drive (third for an auto, fourth for a manual transmission) and shifting into overdrive. A lot of us don't mind pushing our motors a little into the red. With 4.10s 6000 rpm gets you 108 mph. With 4.56s you would be running 97 mph. A lot of naturally aspirated cars trap faster than that.
Almost any power adder pushes the limits of the 4.10/stock size tire in the quarter mile. For example, I trap at 111 mph. At 6000 rpm the speed is 108. I can spin the motor higher, go to a higher gear, or run larger diameter rear tires to water down the gear. A lot of dedicated drag racers choose to go to a larger rear tire with a softer sidewall. It gives them more grip during launches. My car is a daily driver in rain and sometimes snow. I also drive a lot of interstate miles and my T-45 has a lower overdrive gear than the later T-3650s. I chose a higher 3.73 gear.
Lower gears will overall make an auto quicker. But if you are a roll racer they actually make an auto accelerate slower between 65-80. This is because of the large step between second and drive. With stock gears an auto will downshift to second at 65 mph and run up to 80 mph before shifting into drive. With 4.10 gears it can't shift to second at 65 mph because the rpm in second would be too high. The overall gear ratio of the drive train is the transmission gear ratio times the differential gear ratio. Second with 3.27s has a lower overall gear ratio (5.07:1) than third with 4.10s (4.10:1).
So basically it boils down to this. The gear you choose depends upon how you use your car.