JPOR: Tire and Gear Calculations

[kerr@xxxxxxxxxxxxxxxxx]

I'm posting these for no specific reason, just some slightly useful information
to have.

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Equivalent Gear Ratio:
New Gear Ratio = (New Tire Diameter (inches) / Old Tire Diameter (inches)) x Old
Gear Ratio

Example: When changing from a stock 28" tire to 35" tire, with factory gearing
of 3.31, what is the equivalent factory gear ratio?
New Gear Ratio = (35" / 28") x 3.31
New Gear Ratio = 4.14

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Metric Tire Diameter:
Tire Diameter (inches) = ((2 x Section Width x Aspect Ratio) / 2540) + Wheel
Diameter (inches)

Example: How tall is a 325/60R15 tire?
Tire Diameter (inches) = (((2 x 325) x 60))/2540) + 15
Tire Diameter (inches) = 30.35

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Speed/RPM Relationship (assuming a 1:1 final ratio):
RPM = (MPH x Gear Ratio x 336) / Tire Diameter (inches)

Example: How fast must an engine spin at 65 mph, with 4.56 gears and 35" tires?
RPM = (65 x 4.56 x 336) / 35
RPM = 2,845

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-Courtney

kerr@ccmail.nevada.edu
http://www.unlv.edu/staff/kerr
'83 Bowtie CJ-7
'98 Sport TJ