Once again, the Legendary Alex Quirk has graced us with more of his phenomenal knowledge !
THIS WAS TAKEN FROM THE CHEVETTE PERFORMANCE FORUM FROM
SEPTEMBER OF 2000......
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To figure Engine RPM given Tire Size, Transmission & Axle Ratio
use the formula:
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Given 155/80SR-13 tires and 3.62 Axle Ratio and 4 speed
(direct drive 4th)
Tw = Tire Width in mm = 155
Ar = Aspect Ratio in Decimal Form = .80
Rb = Rim Bead Diameter in Inches = 13
Ax = Axle Ratio = 3.62
Hg = Highest Gear Ratio of Transmission = 1
Cl = Clutch Efficiency(should be 100% or 1 if not slipping) = 1
(or less than 1 for an automatic without lockup)
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FORMULA
(5280 /((Tw x Ar x .0787 + Rb) x .2618)) x Ax x Hg x Cl = RPM's
@ 60 mph
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for example above:
(5280/((155 x.80 x.0787 +13)x.2618))x 3.62 x 1 x 1 = 3208 rpm @
60 mph
for 175/70SR-13 tires with 3.36 axle and 5 speed
(.86/1 overdrive) :
(5280 /((175 x.70 x.0787 +13)x.2618))x3.36 x.86 x1 = 2574 rpm @
60 mph
for my former (Plus +1) 14" wheels with same car and 185/60HR-14
tires :
(5280 /((185 x.60 x.0787+14)x.2618))x3.36 x.86x 1 = 2563 rpm @
60 mph
My Solo 2 tires on my '81 215/50ZR-13 on 13x8 rims with 4.11
axle and 4 speed :
(5280 /((215 x.50 x.0787+13)x.2618))x 4.11 x 1 x1 = 3863 rpm @
60 mph
Does this give everyone an idea how much tires and axles affect
top speed, etc. ?
This formula should give you relatively accurate results with
all different axle
ratios and tires sizes...
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Here are some sites that will do the tire ratio, gear ratio, speed, etc calculation for you !: http://www.vmar.com/java/gear.html or http://www.smokemup.com/utilities/calc/rear_end.cfm or http://www.artsource1.com/tritownstreetrods/GearRatioCalculator.htm or http://www.bgsoflex.com/auto.html