Riddle me this transmission man!

[LSDrift]

So I've been doing some research lately on gear ratios and final drives etc. with the RB25 and S13. I've been curious because on the highway at 80mph I'm seeing 3900rpm in 5th gear, which is rather high...I'm still waiting on a Z32 speed sensor so I've been checking my speed via GPS but every RB25/Skyline/Nissan Gear calculator I can find says with my tire and final drive setup I should be seeing 82ish mph at 3500 rpm. Was there a different final drive for the KA24E S13 than the DE version?

The GPS's I'm using are a GPS speed indicator on my droid and my passport radar detector which have always been +-1mph in the past...so I'm a bit confused here...

[240z4u]

What is your source for your tach?

Google how to check what your final drive ratio is. I don't recall exactly how but you can turn your wheels and count the revolutions of the input flange or something of that nature to calculate what gearing you actually have.

[LSDrift]

Ok yeah I'll have to check on that. I've got the S13 DE tach in there adjusted to read pretty close. It's off by like 50rpm at idle which I don't think would translate to a 400 to 500rpm difference at 3500 rpm but I guess I don't know the workings well enough to say. Unless the final drive is different between 90 and 91 I should have the 4.038 open diff. Plan to change it in the next couple months hopefully but until then...

[julio]

*4.083

What tires are you running in the back?

[LSDrift]

Oh yeah sorry 4.083 hah. I think I just figured the problem out here on lunch. Pretty sure it's just my tach . I don't remember what rpm the stock limiter is set for but I hadn't hit it until I tried today and I just bounced it once off at about 7700-7800 rpm. So apparently 50ish rpm at idle can translate to 500ish rpm up higher? I've just got little baby 205/45/17s front and back until I have money for 5 lug hubs, bigger wheels and tires.

Only problem now is that I'm pretty sure that I've got the potentiometer on the tach turned all the way to the left and adjusting it the other way would be counter productive...

[240z4u]

Your tach will never be totally accurate since it is calibrated for a 4 cyl engine. Mine isn't either and that is why I asked. I actually set mine so I would hit rev limit right around the redline mark or a bit higher so I would know right where to shift at a glance. Do some looking around there are others who have retrofitted 6cyl tachs from other cars and those read accurately all the time.

[LSDrift]

Well shoot...I was hoping to just run this tach and it would hit spot on but my OCD probably won't let me not change it out now for something that reads right on even though it's not that big of a deal...

[240z4u]

Search the forum, I am pretty sure it was a maxima tach or something. I recall it being an inexpensive fix.

[LSDrift]

Yeah I've seen people using the Maxima tach on here but I just was attempting to keep it factory looking in the gauge area with them matching and all...oh well, I guess they're nothing spectacular anyhow haha.

[pepegurr]

Any Nissan 6Cyl tach should work. Im running one from a Z32. J30 works too, as well as one from the v6 maxima.

[LSDrift]

Between those three I like the looks of the Z32 one the best, plus I found a whole gauge cluster for 60 bucks. Did you just pull the tach itself from the gauge cluster? Does it have the same 3 connection screws as the S13 one does? Or does that gauge cluster have the same plugs on the back? Thanks for your input everyone!

[platez808]

even if they dont bolt in and take some modification to fit, maxima and i30 tachs look closest to stock in my opinion.. just without the clock

[RustspecS13]

They also aren't any more accurate most of the time then what you have. Its been noted on maxima.org that the tachs are off either in the mid range or top end some times....

So its kinda 5 of one, half dozen of another...

~Alex

[pepegurr]

My Z32 tach is spot on going by the consult. It does have the same three connection screws although it is about .25" deeper than the stock tack so you have to slightly modify the housing for it to sit flush. It does however not match the stock gauges, I went with it bc I picked up the whole cluster shipped for $40.