[Lilce]

My '71 has had the differential replaced, and I'd like to determine the ratio. I did a rough check by turning the driveshaft, and the replacement's ratio is around 4. But since I gather that there are 3.90 and 4.11 differentials, even among R180s, I'd like to be more precise. (According to Transmission and Rear End Gear Ratios, a US R180 from a Z wouldn't be either 3.90 or 4.11, but apparently there are R180s with those ratios from other Datsuns.)

So here's my question: How do I account for the differential action? One of the wheels might move more than the other when turning the driveshaft, since it's not an LSD. Can I count the number of revolutions for each and average them? As I understand differentials, that should work, but I'd like confirmation.

Or would I be better off just jacking up one wheel? Then I think I'd have to divide the number of wheel revolutions by 2 when comparing to driveshaft revolutions. Right?

I also have a posting here offering the car for sale. If someone buys it, I won't care about this, but the speedometer is so far off I want to fix that.

 

[oregonipa]

If you want to eliminate all doubt, crack it open and see what's stamped on the ring gear. This chart is a good reference. Unlike the chart you linked, this one shows what will be stamped on the ring gear and what ratio that corresponds to. The new owner will likely be happy that the diff has fresh oil.

 

[Lilce]

I'd rather not open it up. It already has fresh oil.

 

[bonfire79]

you've got to get the right speedo cog. it needs to match the diff. say you're running a 3.90 rear end... if you got a cog for a 3.54, the speedo will not be accurate.
also, (and I just discovered this a few days ago) if you look on the top of the differential, you might see a number stamp that may give you a clue which diff it is.
I found a picture of a 3.90 rear end for sale and it had a marking number stamped on it.
my 280zx with its 3.90 diff has the numbers "P9000" stamped on the top, same number that I saw on the one in the picture for sale.
I used a mirror tool to look up in there since its installed on the car. makes sense to me, but I could be wrong on what that number is.
don't know if the 240s and 280s had stamped numbers on the diffs, but it might be worth a check.

 

[Lilce]

36 driveshaft revolutions for 17.5 wheel revolutions with only one wheel moving. So 36/8.75 = 4.114 with more precision than process had. Apparently the ring: pinion ratio for a "4.11" is 37:9, which is actually 4.111, so question answered.