R180 3.90 Differential

[Paul_S]

This should help !

The Excel sheet lists a common rev limit of 6500 and same wheel/tyre sizes for both the 3.54 and 3.9 diff ratios.

Play around with it with YOUR rev limit wheel and tyre sizes AND and in the different diff rations to see what adds up to what and THEN choose the 'best' for YOUR own use and application.

Second Excel sheet is a compilation of diffs and gearbox ratios that I knocked up with info from here and there for our members here wanting to convert from US import specifications*.

*there may be errors and I'd be grateful if you'd point any out so as to be corrected.

Enjoy, discuss but don't let anyone tell YOU what is 'best' for YOU......remember, "you're all individuals - you've all got to work it out for yourselves"..

Good info there!

I've been reading this thread with great interest. Thanks all

 

[samuri-240]

Could anyone tell me what ratio would have been fitted to a 1978 uk supplied 260Z 2 seater please.

 

[samuri-240]

Sorry it may have been a 1977 260Z, well we call them late 260's anyway.

 

[SeanDezart]

Could anyone tell me what ratio would have been fitted to a 1978 uk supplied 260Z 2 seater please.

Can you not open the Excel sheet ?

260Z 1977-8
Euro
FS5W71B
3.321
2.077
1.308
1
0.864

1:3.7
R200

260Z EU

 

[jonbills]

...which seems pretty reasonable vs the nearly 4k I’m sitting at with the current 4 speed!

I think you might want to check on what diff ratio you actually have fitted in the car, as - reading between the lines somewhat - it seems to me that it doesn't sound like the original 3.3 ratio...

I checked this in mine today, which also has the 3.36:1.
70 (GPS measured) is 3,500 rpm in 4th and 3,000 rpm in 5th.

 

[johnymd]

I'm surprised your running a 3.36:1 Jon. Have you ever thought of swapping it out to improve acceleration?

 

[Albrecht]

I'm surprised your running a 3.36:1 Jon. Have you ever thought of swapping it out to improve acceleration?

Indeed. What's a 3.364:1 R180 doing in a UK market RLS30?

 

[jonbills]

The roll pin sheared in my original 3.7 r200 diff last year, so I bought this, allegedly from a z31 NA manual which should have been 3.7 or 3.9. But it ain't
I fixed the 3.7 but haven't put it back in - I'm thinking of putting a Quaife in and also procrastinating.

 

[Woody928]

I think you might want to check on what diff ratio you actually have fitted in the car, as - reading between the lines somewhat - it seems to me that it doesn't sound like the original 3.3 ratio...

Also bear in mind that numbers on paper are all very well, but it's what you feel in the car (the old 'seat-of-the-pants dyno') that's most important to you. Gearing will always be a compromise in many respects.

Also bear in mind that R180 ratios ranged from 3.3:1 right up to 5.1:1, so - all things considered - the recommendation of 3.9:1 is a fairly conservative, intermediate one.

I’ve not checked it against a satnav and I wasn’t driving however I got told that at approximately 70mph it was 3750 rpm in fourth from a test over the weekend. So it’s fair to say I would generally cruise at near 4k rpm, is there any way of telling what diff is fitted without removing it?

I’d definitely agree experiencing something first hand is always best however that’s easier said than done finding two vehicles of similar spec with difference diffs for comparison…. A 3.9 sounds about right then as an ‘intermediate’ choice given I want to balance on road performance vs the ability to comfortably cruise. Any offers of a passenger ride gratefully accepted

I checked this in mine today, which also has the 3.36:1.
70 (GPS measured) is 3,500 rpm in 4th and 3,000 rpm in 5th.

Thanks for sharing

 

[Rob Gaskin]

.......... is there any way of telling what diff is fitted without removing it? .............

You could try finding two mates, jacking the rear off the ground, marking both wheels and propshaft with chalk, then counting the difference in revs.

So turn the rear wheels together (no diff action) for say 10 turns and get a mate under the car to count the turns of the prop.

So if the prop turns 37 times you have a 3.7.

Ok it's a pain but you did ask.

 

[jonbills]

Assuming original tyre circumference and correct speedo drive gear in the gearbox, I'd bet an indicated 70 is no more than 65.
So that would be about 4k rpm at a real 70.
I think that may well make your current diff at least a 3.7, maybe even a 3.9.

 

[Woody928]

Thanks chaps, I think what I'll do to start with is when I'm next out in the car I'll get the satnav out and get a gps 70 mph and then look at the rpm so we've got a genuine gauge on whats going on there and then assuming I'm really struggling then I may go all out with Rob's rem methodical approach

 

[Paul_S]

I have a question, and it's related to this so I'll ask it here...

How do you go about re-calibrating the speedo having changed over the diff? Or do you just live with it being out?

 

[jonbills]

You can buy from Mike F a speedo drive gear for the gearbox that matches your diff ratio.

 

[Paul_S]

You can buy from Mike F a speedo drive gear for the gearbox that matches your diff ratio.

Excellent answer. Thank you