Re: 8 1/4 rear end


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Posted by Chewie on Wednesday, July 15, 2009 at 13:17:05 :

In Reply to: 8 1/4 rear end posted by sean on Wednesday, July 15, 2009 at 10:35:24 :

Suggestions? Figure that 1900-2100rpm is the best cruising speed. Whether that's 55 on blacktop, or 65 on the highway will make a difference. And an overdrive tranny will make a difference, too. But here's what's out there:

Divide 336 by your tire diameter:
336/31"= 10.83 /33"= 10.18 /35"= 9.6
Use that in this equation:
Gear X MPH X Tire = RPM
3.55 X 55 X 10.83 = 2114
3.55 X 55 X 10.18 = 1987
3.55 X 55 X 9.60 = 1874
4.10 X 55 X 10.18 = 2295
4.10 X 55 X 9.60 = 2164

This isn't an exact science, as loaded diameter will always be less (and therefore give a higher RPM), but it gets you in the ballpark. The formula for figuring out the diameter of a metric tire is:

Width X % / 2540 X 2 + wheel diameter= diameter
235/75R15 = 28.87" diameter
235 X 75 / 2540 X 2 + 15 = 28.87"
245 X 85 / 2540 X 2 + 16 = 32.39"
336 / 32.39"= 10.37
3.55 X 55 X 10.37= 2024rpm

If you have an OD tranny, put that into the RPM formula: (NV4500 has a 0.73, or 27%)
3.55 X 55 X 10.83 X 0.73= 1543rpm
1543 is low, so you could move up in gearing.
4.10 X 55 X 10.83 X 0.73= 1782rpm
4.56 X 55 X 10.83 X 0.73= 1982rpm
My suggestion is 4.10s with 35" tires. That will give you 2200 or so at 55, and enough gear to pull with. Just a suggestion.
Chewie, layed off for the moment.



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