what is my final gearing?

[kmack]

Well, since I finally got to drive my car for the first time to work today, I have a question.

The PO has told me that at one point they had put a 5-speed in this car. I can't get him to confirm whether or not they replaced the differential or not, though. Which leads to my question.

I have the original 4-speed (A-Type) in the car. 215/60/r14 Yokohama S306 series tires. At 75 mph, the rpm's are at 4100. 60 mph is around 3200 rpm, but I can't confirm that reading like I can the 75 mph one.

Anybody have a way to figure out what my final gearing is in the differential? Car is a '71 Series I if that helps.

[V8-240Z]

Give this link a try http://www.prestage.com/carmath/calc_gears.asp

[Royce]

Hi kmack,

Using 4th gear is a good way to figure this out since it is always 1:1 in all the standard Z transmissions. What makes this difficult is that we don't know how accurate your speedometer is! You really need to find out how fast you are really going at that speed to figure it out this way.

I put your tire size, desired RPM and checked a few different rear end ratios, according to http://www.geocities.com/z_design_studio/

4th Gear 4100 RPM

Diff Ratio MPH@4100RPM

3.364 86

3.545 81

3.7 78

3.9 74

4.11 70

Now there is another way, the junk yard technique that will probably save you some time and effort. You need to spin one rear wheel while the other is still, just jack up one side of the car. Put some jack stands under there and put an easily visible mark on the drive shaft and on your tire. Now rotate the free wheel 10 times and count the number of rotations of the drive shaft. If the drive shaft spins less than 17, then its 3.365; 17.5 then a 3.54; 18.5 then a 3.7; 19.5 then a 3.9.

By the way I borrowed this technique from www.zhome.com were it was documented by our very own Ross Corrigan. I have used the technique and it works quite wheel!

Have fun!

[kmack]

I assume the speedo gear in the tranny to be the stock original for this car. And it looks like I may have a 3.90 then. I'll jack it up and check this out soon to make sure.

Thanks for the help.