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Gear Ratio Determination

#1 User is offline   Jim B 

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  Posted 12 November 2004 - 09:13 AM

If you are not sure on what gear ration you have, there is an easy way to determine this instead of going to a shop as saving a bit of money here and there always helps. I posted this info on a prior thread, I'm posting it on it's own so we do not loose this simple trick that might help someone in the future.

Jack up your rear axle, make sure it is up in the air with the drive shaft disconnected & neither wheel on the ground:

If you turn one tire & the other one either stays still while the pinion rotates, or the other one rotates backward while the pinion stays still, or something in between, then you've got an open diff or a very weak Limited Slip. In this case, you need to secure one wheel (lower the tire to the ground or wedge it against something), you will need to do this to figure out your gear ratio.

If you turn one tire & the other one turns the same number of turns in the same direction, then you've got a good Limited Slip or locker. In this case, you can just rotate the tires.

Figure out your gear ratio by rotating the airborne tire TWO full rotations and counting the number of times the pinion rotates. 3.73 turns means 3.73 gears. If you only rotate the tire once, then 1.865 turns means 3.73 gears.

The above is an easy way of doing it.... or you can knock yourself out by removing your diff cover.

Rear end numbers are the ratio of ring gear teeth to pinion gear teeth, which tells how many times the driveshaft turns for one revolution of the differential housing.

Number of Teeth on Ring Gear divided by Number of Teeth on Pinion Gear will equal your Ratio.
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#2 User is offline   Mountain Man 

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Posted 12 November 2004 - 12:32 PM

There is an easier way.....without dropping the drive shaft.
Put a chalk mark on the tire, at the top of the tire on the sidewall, jack up the rear of the vehicle, put safety stands under the vehicle, put the vehicle in neutral, put a mark on the driveshaft. Have a buddy turn the tire 10 times (clockwise in the northern hemisphere).....and then count the number of times the mark revolves around the driveshaft. It will be something close to a known axle ration (3.54, 4.11, 5.38)...saves all that hard math.
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