There is a logical science to this.
When racing with nearly unlimited traction you want the car to go through the lights at maximum possible safe RPM in the highest drive gear. That would NOT normally be overdrive.
When you are traction limited a lower gear (taller ratio) can actually slow you down. With a manual, especially with the T5's wide ratio for 1st, you may not want such a steep gear.
My car is a good example. It had 4.10 gears with a T5 when I bought it. Running off the basically stock motor (except for GT40 heads and induction and headers) and 245/ 60/ 16 tires, there was no reason to use first gear. I could start in second and run the same ET as starting in first, and I went through the traps at 5500 RPM or so. This is safe for the motor and what it needed except I could have used much less gear and used 1,2, and finished in 3rd.
My 89
trans has 1st 3.35 2nd 1.93 3rd 1.29 4th 1.00 5th .675
So a 3rd gear finish that would equal the 4.10 would require 1/1.29 = .7752 times 4.10 = 3.18 gear. If I had a rear gear around 3.18 the car would be just as fast except I would use 1,2 and 3.
When I added a cam, heads, intake, and much stickier tires the car was finishing at 6000 RPM in fourth. Now 1st gear mattered. So I needed the 4.10
Now that I added a blower the car goes 7000 RPM in fourth before finishing the 1/4 mile. Now I have problems. I have to cut about 1000 RPM out of the finish-line engine speed at MORE MPH (because it has never reached the real top end because it has too much gear).
My best estimate is a 6000/7000 = .857 times 4.10 = 3.51 gear will get me at 6000 RPM about 100-150 feet before the traps. I am 7000 RPM there now with the 4.10, I want to be 6000 RPM there so hopefully it will be about 6500 (cam maximum RPM range) at the finish line.
The ONLY guess in this is how much RPM the engine will gain in that last 150 feet where I now have to coast.
I had three choices. I could have used a taller tire, but it would take 27 * 1.167 = 31.5 inch tall tire. That won't work.
I could buy a transmission that would let me power shift into overdrive, and it would have to have a 6000/7000= .86 overdrive and that would only be used for the last 150 feet or so. My car is traveling 176 feet per second at the end of the 1/4 mile (FPS is MPH times about 1.47), so if I shifted at 6500 RPM I would be in overdrive for only about 1.5 seconds to 2 seconds maximum. Hardly worth the abuse on the
trans or clutch.
I could go to a 3.55 gear and leave everything else alone and finish in fourth. This would give me more distance and time in first, second, and third. I wouldn't be rushed to grab 2nd.
I could also use a higher rear gear (less ratio) and a taller tire. If I went with or could find a 28.5 inch tall tire I would gain only 100* (27/28.5)= 94.74% gear reduction.
I need (6000/7000)*100=85.7 % of what I have now. So that is 10% short. .9 times 4.10 = 3.69 : 1 gear. I could use a 3.73 and a 28.5 inch tall tire.
With an automatic I would have to figure in torque convertor slippage at the end, so it probably would require LESS than a 3.55 gear or a much taller tire if the car went the same speed.
The gear you pick really depends on the traction, the MPH at the end of the track, the red line RPM (you want to be past the power peak but safe), and the tire size.
Another reason I'm going to the 3.55 gear (with Cobra differential and 31 spline 4 lug axles)? I plan on making the car faster. I want a ten second street car, and that will mean 125 MPH or so in the 1/4. I can always hammer out the wheel wells and add a taller tire later to cut the gear back some more when the car goes faster, or I might just build a higher RPM motor so it runs through at 7000 or more. For right now though, there is no debate for me. It is all in the numbers. If I want to run 120 MPH and have a tire fit the wheel wells, I MUST install a 3.55 gear. There is no other logical choice.
Tom
3.73 rule! 
