How to determine gear selection?

[OIF1VET]

I know there is a formula out there to predetermine what the optimal gear ratio is for your car when drag racing, based on tire size, etc.. Does anyone know how to check this? Reason I ask is last night I was dinking around(no traffic around) and took it up to around 110 and my shift light came on(I was in OD). I thought that was odd, because with the 18" rim/tire combo I was a couple hundred rpms short of redline in 3rd gear at the strip and trapping 108-111. Below is a chart with my 2 tires I run, #1 is the 18's and #2 is the 15's. I don't see any reason my gear ratio should of been effected that much.. Maybe it was just a fluke???

Result Totals are in inches
Sidewall Height: 1. 3.93 2.5.41
Section Width: 1. 11.22 2. 10,83
Overall Diameter: 1. 25.85 2. 25.83
Circumference: 1. 81.22 2. 81.14
Revs per mile: 1. 804.64 2. 805.27

Speed on 18's 65 MPH, Corrected speed on 15's 64.95 MPH

[Swat]

.020" isn't anything. Sounds like a fluke. I only know the formula to determine RPM @ a given speed with a given gear ratio and tire dia. (not sure if that's what you are looking for)

336 x gear x speed (x .7 if determining in O.D.) / tire dia.


example 1:1 for 3rd gear

336 x 3.73 x 75mph / 25.83 = 3639 RPM

example .7 for O.D.

336 x 3.73 x 75mph x .7 / 25.83 = 2547 RPM

[OIF1VET]

.020" isn't anything. Sounds like a fluke. I only know the formula to determine RPM @ a given speed with a given gear ratio and tire dia. (not sure if that's what you are looking for)

336 x gear x speed (x .7 if determining in O.D.) / tire dia.


example 1:1 for 3rd gear

336 x 3.73 x 75mph / 25.83 = 3639 RPM

example .7 for O.D.

336 x 3.73 x 75mph x .7 / 25.83 = 2547 RPM

Yeah I didn't think so either.. Wanted to keep it close to what I had.. Wierd that that happened.. I guess we'll see when I get to the track! Thanks!!