Can somebody explain me the difference between 110 and 115mm.

I’ve got a 180 diff 3.9 and love to know if this a 110mm ring or a 115mm.

Does a 115mm rack and pinion set fit in a former110mm? Vice versa?

The ID of the ring gear changed sometime in the mid 70s. Most of the aftermarket LSD's are 115 mm, so if you have the 110 they are too large to fit into the smaller ring gear. BTW its ring and pinion, or crown and pinion, not rack and pinion. I suppose on your 3.90 you'll have to pull the ring gear off and measure the ID to know what size it is.

3.90, 4.11, and 4.38's are all common ratios from the front of 80's Nissan 4x4 trucks here in the US. I don't believe any US version Z car ever came equipped with a 3.90 R180. So if you were in the US I would doubt that it is the 110, unless some early owner of the car went and bought 3.90 gears from Nissan Comp. Europe was probably different. You got 5 speeds, so it would make sense that you got lower diff ratios as well.

Well, that is an expensive way to find out. Take the diff apart, measure it and bring the diff to a workshop to get it in one piece again! I mean to adjust ring and pinion is specialist work!!

The reason why I ask this is; I have an option to buy a very low ring gear.

Quote: “They were originally purchased in a brand new, pre assembled competition LSD R180 setup direct from NISMO as advertised in their catalog. The ring gear in this set has the inner machined recess measuring 110mm in diameter.”

Actually pulling the carrier out of a diff is very easy. As long as you keep the shims straight it goes back in easy too. Getting the diff in and out of the car is the hard part, but I guess I assumed you had one already out of the car. The hard part of setting up gears is setting the pinion depth, but in order to take the ring gear out removing the pinion and it's associated shims is totally unneccessary. But that's a moot point since you apparently know the history of the gears.

So the answer to your question is that the 5mm smaller ID makes no practical difference as far as the strength of the gears and all that, so there is no reason to avoid or prefer one over the other.