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.

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.

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.

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

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.

okay, looks like concensus is that my math is correct. Looking for an NB 4.3.

If you want to post up your speed and RPM per gear, I would be happy to figure out all the trans ratios.