4 turns of the diff yoke gets me almost 1 full turn of the axle.

I know the stock rearend is 3.90 - is that what this is?

Without pulling it apart is this the only way to tell what the diff is?

thanks, Evan

To calculate an open diff ratio whilst in the car do the following:
The car must have both rear wheels on the ground,place a chalk mark on the tyre at 12o'clock,place a chalk mark on the tailshaft any where you can see it,have a mate push the car one full revolution of the wheel whilst you count the revolutions of the tailshaft,you will end up with 3.9 turns or 4.1 turns of the tailshaft,any more or less turns and you have an unusual ratio.This method must be employed if spider gears are opperational,if the diff is locked,jack it up and simply count revolutions as you have done

What vehicle do you have?
Do you believe that it has the original differential?

We [the forum] know what the original ratios were in most Datsun models in most parts of the world, so if it's the original, in a model that's known to this forum, we should be able to come up with a number for you.

Calculate it like Bert described. There is no point guessing by someone telling what it should have.

The diff is out of a 110 coupe (N. America), I believe the stock unit should be an H145 w/a 3.90 / 1 ratio. thats what my factory manual spec's say & whats stamped on the gears of the unit I'm keeping.

This one is out of my parts car I'm working towards getting rid of.
I had the whole rearend unit out of the car sitting on a garbage can when I checked this. I marked the yoke & housing as well as the axle & backing plate & then counted the turns of the axle end to the one turn of the yoke. I'll have to do it again because I cant remember if it came up slightly short of the housing mark or slightly past it.

I'm assuming if it's slightly before my backing plate mark it's a 3.90 & if it's slightly past the mark it's a 4.10, would this be correct?

I'm just trying to figure out if I should tear it apart & keep it as a spare or let it go down the road with the shell. I'm getting sick of pulling & swapping parts.

I've never seen a 4.11 here in USA. That was a ratio used in other countries for Datsun 1200 Truck or Wagon.

Anyways, just turn it 10 times to see if it goes 41 times or 39 times.

For the rear axle assembly, I wouldn't bother to tear it down. Just re-pack the bearings and ensure the brakes are good. Later on -- since this is a pumpkin type differential -- it is easy to swap the center section if needed. Most 1200s have original differential with no problems. They don't usually wear out.

Cannot be calculated accuratly whilst in the air!
I sometimes wonder why i bother!

If the diff is from a Datsun 1200 that was sold new in the US, then it will be a 3.9 to 1 ratio. There was a seldom seen 4.11 to 1 ratio that was optional in Station Wagons, but the US never got these, so as long as you have a H145 Datsun 1200 rear end with the iron differential carrier [the pumpkin] then it'll be 3.9 to 1.

Should you let it go? Well only you can answer that. With a stock engine, the diff will last almost forever, but with some abuse behind a more potent engine, the service life can be severely shortened.

If you have the room to store it, then do so as there will inevitably be someone who needs it down the track.

OK, the answer is....3.90! I pulled it out, stuffed the axles back in the housing & rolled it out the door. I contemplated keeping the axles too but couldn't figure out an easy way to keep it a roller so someone else, likely a tow truck, can pick it up.

I'm curious - Burt said "cannot calculate accurately whilst in the air" - why not?

what difference is it if the wheel is turning on the ground or the axle end turning in the air, it's still turning the same # of revolutions.

Anybody want a partial rolling chassis? Free!! DD, your close, the wife would be really happy to have another 1200 at the house!

Evan,rather than me pounding away for hours here..Google Diff spider gears and read till your hearts content.
Cheers Bert