Transmission ratios and suggested rear gears

[2ManyZs]

If you are sure the rear differential is stock, check to see which drive gear is in the transmission. If it is not the 16 tooth yellow drive gear you will need to change to that to make your speedo right. I am assuming the car came standard with an manual transmission, therefore it would have a 3.36 rear, if it were an automatic it would have a 3.54, since you are not sure what is in there you should check to see what ratio you have for sure. You can find posts here using the search function that will tell you what to do to find out the rear ratio without tearing apart the diff.

As far as the speedo cable goes, you should be able to find one in the junkyard off a 240 with no problem or you might try to buy one form someone stripping a parts car. If you want a new one you can visit www.zcarparts.com, request a catalog or call them directly. I'm pretty sure Motorsport Auto will have a new cable, although you may want to look around before you spend the money for a new one.

[carguyinok]

http://www.geocities.com/zgarage2001/engine.html

read away good tips & pics

[Hallaian42]

Quick question...I have 79-83 block with n47 head and a 5 speed in my 240...i believe i have the stock 240z differential...Right now i have good acceleration but my 4th and 5th gear have high rpm's and I can really cruise on the highway with out high rpm's...What would you suggest i get to have really good acceleration but having good cruising rpms on the highway? would a r-200 with 4.11 work? And where can i find one? Thanks

[Hallaian42]

I CANT cruise on the highway without having high rpms ...sorry:stupid:

[Royce]

First, a 4.11 diff would make your car rev more on the highway, assuming you do have a stock rear end. Going to a R200 will provide you a potentially bullet prof diff but would also add somewhere close to 20lbs to the weight of your car. Not to mention it will require a little more work to swap in than another R180. It isn't hard but it is something to be aware of.

Second, would be good to know more about the car!

What do you consider "high RPMs???" Ex. 4000RPM @ 75MPH

What 5 speed do you have now or how much do the revs drop when cruising and 65MPH? EX. 500 or 1000 RPM.

What size tires do you have and are you planning to change your tire size?

I have a hard time thinking your RPM would be that high at freeways speeds unless your diff was changed, your tires are very small and low profile or you are used to driving a V8 that runs down the freeway at 2500RPM.

Have fun!

[Hallaian42]

Thanks for the info royce...you have to bear with me im new to these cars. The bottom line is I can hardly feel a difference between 4th and 5th. I feel like it doesnt matter if i have it in 4th or 5th on the highway.... The RPMs are about 4000 @ 65....Thats just a guess becuase i dont have a working RPM or SPeedometer yet!!! So would you suggest maybe a different r-180 with just different gearing or can i change the gears in the one i have now? Thanks again

[Royce]

Hallaian42,

From what you describe most likely you have a 280Z 5-speed. I say this because I have one and the drop in revs from 4th to 5th is only about 400-600RPM at freeway speeds. It doesn't feel like they drop much at all if you are working through the gears as you accelerate. If you are revving as much as you think you are than your diff was probably also changed to a lower rear end, higher ratio (3.9 or 4.11). There is an easy way to tell what your diff ratio is. Jack up your car from one side so that one rear tire is off the ground and the other is still on the ground. Support the car on jack stands, but you knew that, and put the car in neutral. Mark a visible spot on the drive shaft and on the tire that is off the ground. While some turns the tire 10 times count the number of rotations of the drive shaft. I think the relationships are:

rotations..ratio

..16....3.36

..17....3.54

..18....3.7

..19....3.9

..20....4.11

etc.

There is a technical writeup on this method at www.zhome.com under "z technical library"/"Index of technical articles"/"How to determine what ratio a specific differential contains" by Ross Corrigan.

Then once you determine your diff ratio, you can go to http://www.geocities.com/z_design_studio/

and plug in your diff ratio, trans "77-79 Factory Five Speed" is a preset, and your tire sizes and it will calculate for you what your RPM is at various MPH. So you can find out what your RPM is at 65MPH without a tach.

Have fun!

[Bleach]

10 turns of the tire gives you X driveshaft rotations

DS rot. Ratio

16.9 - = - 3.36

17.5 - = - 3.54

18.5 - = - 3.7

19.5 - = - 3.9

20.x - = - 4.111

21.x - = - 4.375