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Discussion Starter #1
i assumed it was stock, but when im going 65mph in my 72 w/ a 4speed (L28 powered), my tach is at 4k. this seems higher than it should be. do you think its a new rear gear? if so, how do i find out which one? thanks
 

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I think that's a stock rear end. Mine ran high revs when it had a 4 speed as well. You can put a 5 speed, but then your "off the line" take off would suffer. The stock ratio is around 3.35:1.

I don't know if this will work with the tires on the car but it's worth a try.

I've done this in the junk yard. Mark all three flanges with whiteout or chalk. Grab the input side of the diff (the one that points to the transmisson) and turn it. Watch for one full rotation of the wheels, and count how many rotations at the input shaft. on a 3.35:1 the input shaft will go around roughly three and a half times for one full rotation of the wheels. On a 3.91:1 it will go almost four times around. Make sense?

I've go a 3.91: with a '78 5 speed and freeway revs are slightly better, but off the line is great!

Hope this helps.
 

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Discussion Starter #4
thanks for the link gearhead, its very useful. but it shows that if the diff is stock, i should be at about 3k rpm at 65. Even when i try a 3.9, at the same speed it would only be at 3500 rpm! i have stock wheels/tires. could it be a 4.1? well, thanks.
 

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You can enter custom ratios

Use the claculator and just enter your own ratio of choice like 4.11 and it will work, you don't have to stick with the pull down options that were only for stock rear ends. The Z never had a 4.11 in any version, but motorsports has the gear sets and apparently othe Nissans as well. Seems to be a popular mod for autocross and drag racers (light to light). Try it with your wheels and tranny see if it comes out right.
 
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