10-19-2012, 06:27 PM
Join Date: Jan 2010
Total Cats: 14
Differential gearing, help me check my math
I have never claimed to be the sharpest when it comes to calculations for speed, gearing, tire circumference and diameter.
I went to 949's site and found their formula for gear ratio. I would like some verification of my math and thought process. I am using an alternative transmission and am unsure of the ratios in the transmission. From my data acquisition system I know that my current setup: 4.77 gears and 22.9 diameter tires nets 130mph at 8k rpm.
I think that the below formula is based on 1.1 in the transmission
According to 949, gear ratio = (rpm x tire diameter) / (mph x 336)
in my application (8000 x 22.9) / (130 x 336) = (183200) / (43680) = 4.2(rounded) this is for my current setup. I know that I have 4.77's in the diff currently so this means that my transmission has an overdrive 5th gear.
Calculating for 13 inch tires
(8000 x 20.8) / (130 x336) = (166400) / (43680) = 3.80
This means that I need a rear end gear that is 110% of my current gear (4.2) / (3.8) = 1.1(rounded) If my current gear is 4.77 then (4.77) / (1.1)
Is my thinking correct?
Any help is greatly appreciated.