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Old 05-27-2004, 09:58 PM   #1
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DennyB's Avatar
Join Date: Jan 2001
Location: Toms River, NJ, USA (Transplanted Hoosier)
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Lightbulb How to on axle ratios

Hey, This question comes up alot and its alot of typing so I thought I would put it here...

The most accurate way to tell the ratio is to remove the inspection plate. There are numbers stamped on the ring gear. Like 48 13. Divide the small number into the large one and that would be the ratio.
Example" 48 / 13 =3.428 IE a 3.43:1 ratio.

Sometimes there is a tag on one of the cover bolts with the ratio on it. This could even be on the front axel(with a 4WD).

Another way is to do this is....
With an open rear (non posi)- jack up one rear wheel (please use jack stands), put the truck in netural, turn the rear wheel and count how many times the drive shaft turns.
Multiply the turns of the drive shaft x2 and divide for the ratio. This is most accurate if you turn the wheel about 10 times.
With a posi rear, jack up both rear wheels and turn the same way as above. use the actual drive shaft revolutions with a posi.

With both wheels off the ground a posi will spin both wheels the same direction.
With an open rear the wheels will spin opposite directions and also do so easily with truck in park.

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Last edited by DennyB; 10-02-2004 at 12:19 PM.
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Old 11-09-2004, 11:24 PM   #2
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Location: Butler PA
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with one wheel up spin it twice and count the driveshaft rotations and that's the ratio...
67 c-10, 400 SB, TH350, 4.10 gears, Blazer tank,camaro tach, sidemarker fuel fill (soon)
WTB: stepside parts, 67 front end parts, 67 small window cab. posi for 3.73 or 4.10 for 12 bolt and 10 bolt.
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Old 05-30-2005, 03:50 PM   #3
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If you have an accurate tachometer and an accurate speedometer, and either a manual transmission or an automatic with a torque converter clutch, take your truck for a drive and cruise at a constant speed to see what your engine RPM is at that speed. Then you can use the following equation to calculate your gear ratio:

Gear Ratio = Engine RPM x Tire Diameter x .002975 / Speed / Transmission Ratio

If you have a 700R4 Transmission, use a value of .7 for the transmission ratio. If you have a 1:1 high gear, just use a value of 1. If you have an automatic transmission without a torque converter clutch, then this equation will get you close, but the result will be a little higher than your actual gear ratio.
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