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I can't seem to identify any 3:90's at the yards. Popular website states ALL 81 thru 83 non turbo's have it, they're also supposed to have the white 19 tooth speedo cog. Every 81-83 ZX I tried had the black cog indicating 3:54 gears. What part of my info is wrong, the cog color, the non turbo part, or the years of the cars. Pulling the diff cover is really not an option at the yards. thanks for any help
 

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Theres an easier way to figure the ratio but you must pay attn. or all is lost,1st.your gonna need somthing to mark the pinion with and rotate it to the 12 o'clock position.2nd.you must stop one side of the rear end or just have a friend hold it from turning,now mark your hub or wheel if still installed at 12 o'clock.3rd. now you turn your wheel or hub 1 full turn and as you do this count the revs that the pinion makes, if the pinion turns almost 4 whole turns then u have a 390 gear,just over 4 turns its gonna b a 411and if 3 1/2 turns 350s, I truley hope this helps and you might want to print this and take it w/you.
 

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Actually if you hold one of the wheels still, you'll have to turn the other one 2 complete revolutions.
Yes, this is a simple and fast way to check.

I got one from a '85 300zx na with a 5 speed.
 

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I disagree w/that because the ratio is based on 1 to 1,meaning 1 rev to 1 rev is dirrect drive,if 4 turns on the pinion turns the wheel 1 rev. than that is a 4 series gear and the only reason to stop the other wheel is to prevent the spider gears from absourbing any of the revs. If u r testing a posi u don't need to stop the other wheel w/this test.
 

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I think nismotony is correct. Either turn one wheel 2 turns or turn both wheels 1 turn. The shaft will turn relative to the average of the two wheels.
SoM
 
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