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Old 10-19-2012, 07:27 PM   #1
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Default 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)
= 4.33

Is my thinking correct?

Any help is greatly appreciated.
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Old 10-19-2012, 07:44 PM   #2
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I posted in your other thread with a slightly different calculator using the tire circumference as opposed to diameter. Different way to skin a cat.

Test new wheel and tire combo in the calculations to see what is the max mph. If the mph is acceptable, then you may not need to change the ratio.

I also suggested a slightly taller tire for your 13" wheel. If it was a stupid suggestion, forgive me. I am not yet road racing.

Last edited by miata2fast; 10-19-2012 at 07:56 PM.
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Old 10-19-2012, 07:55 PM   #3
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No worries, that is what this forum is about, sharing information. The diameter of the tire that I listed is from the spec line at the Hoosier web site, so that is what is available to me.
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Old 10-19-2012, 08:22 PM   #4
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404, FM gearing calculator not found.
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Old 10-23-2012, 08:55 AM   #5
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Quote:
Originally Posted by hustler View Post
404, FM gearing calculator not found.
It won't help him. Custom gears and custom R&P.

OP,
I think your math is correct. I know it's 4x4 centric, but it works for us too: Gear Ratio Calculator
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Old 10-26-2012, 10:28 AM   #6
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Did some math and got a 5th gear of .879 and your calculation for a 4.33 diff seem correct. A way to insure accuracy is to measure the loaded circumference of the tire. Do this by rolling the car so that the tire makes a complete revolution and measuring the distance traveled.
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Old 10-29-2012, 01:29 AM   #7
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okay, looks like concensus is that my math is correct. Looking for an NB 4.3.
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Old 10-29-2012, 09:50 AM   #8
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If you want to post up your speed and RPM per gear, I would be happy to figure out all the trans ratios.
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