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Discussion Starter #1
Hello all! I have looked all over the internet for a formula that will tell me what rear gear is ideal to hit a specific rpm in the 1/4 mile, with no luck. I had my car dyno'd, and I know that the hp peaks around 5400 rpms. Correct me if I'm wrong, but ideally you want to be at (or just past) your peak hp rpm at the end of the 1/4 mile right? IS there a formula to figure out what gear will put you at a desired rpm or are there too many variables to do that?
 

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If you know what mph you're going to hit then you can figure it out:

Rear gear ratio = (E * PI * D * 60) / (M * 63360)
E = Engine RPM
PI = 3.14159...
D = Tire diameter in inches
M = MPH

This assumes the transmission ratio is 1 : 1 (e.g. 4th gear in a T5).

As an example:

E = 6000, D = 26 inches, M = 124:
Rear gear ratio = (6000 * PI * 26 * 60) / (124 * 63360)
Rear gear ratio = 3.74 : 1

You really want to maximize the average horsepower over the 1/4 mile. That means you want to shift (and cross the finish line) at a point beyond the peak horsepower. And the shift rpm actually changes for each gear because the gear ratios in the transmission are not equally spaced.
 

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Discussion Starter #3 (Edited)
I know there are alot of variables to rear gear selection. I haven't been to the track since the upgrades so I'm not sure what the e.t. or m.p.h will be. I used to have the stock T-5 trans with the 3.55's, but with the T-5 Z trans (taller first gear) I figure I will need a lower rear gear to compensate for that. Plus, the stock setup peaked out around 4500 rpm, where as my new setup peaked around 5400. My guestimate is either 3.90's or 4.10's will fit the bill. The car is street driven with occasional trips to the track for fun.
 
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