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Old 01-05-2008, 22:44   #1 (permalink)
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Recommend diff ratios

Hi iv got a xc falcon with a 400hp 351 and 4 speed wide ratio toploader. Its currently got a 2.75 ratio which is way to tall. I'm wondering if any one can recommend a diff ratio that would be better suited. I only use it on weekends but I dont want it to be to drag like.
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Old 02-04-2008, 10:27   #2 (permalink)
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Re: Recommend diff ratios

I'm guessing you want to be around 3.54 a balance between off the line and top speed. I wouldn't recommend anything lower unless you plan on driving in a straight line or you completly updated the suspension.

Use this formula to figure out were you want to be

Maximum MPH in each gear:

Formula: MPH = rpm x tire diameter (in) / gear x final gear ratio (diff) x 336
Example: Maximum mph in 280Z 5spd 1st gear using 6500rpm redline.
MPH = 6500rpm x 24.0"(tire dia.)/ 3.321(1st gear) x 3.54(diff gear) x 336 = 39.5mph
Note: Top gear is most likely hp limited, and would not be able to reach max rpm.

RPM @ 65mph (Derived from Max MPH formula):

Formula: RPM = mph x gear x final ratio x 336/tire diameter
Example: RPM at 65mph for stock 280Z 5spd
65mph x 0.864(5th gear) x 3.54(diff gear) x 336/24.0"(tire dia.) = 2783 rpm
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