R32 Rb20 With Rb25 Box, Speedo Working But Incorrect.

[CtrldKilla]

Hey Guys,

I bought an R32 GTS-T that has had a RB25 box put in it sometime in the past.

Now I have done some reading and know that you need to convert the electronic speedo to a mechanical one in order for it to work in the car. I'm assuming that this has been done in my car since the speedo is working, however the speedo is out by about 25kph at 100kph. I'm just wondering what you guys think is causing this?

My uneducated guesses are;

The Diff?

Maybe something to do with the mechanical speedo conversation?

Also the car has a GTR dash cluster (could this make a difference?).

Are any of my guesses close? where should I start? I would like to get the speedo as close as possible (doesn't have to be perfect).

Cheers in advanced,

Stuart

[GTSBoy]

Here's the list of things that can affect it.

R32 GTSt diff ratio is 4.3:1. RB25 turbo diff ratios are 4.11:1. Difference of 4.5% which is not enough to explain your error.

R32 GTR diff ratio is 4.11:1, like the RB25 diffs, so that could/should get rid of the 4.5% error above anyway.

BUT....and here's the big but.....The GTR speedo expects to be turned at a particular speed by its speedo cable. That speed is dependent on the number of gears on the drive in the box and the end of the cable. The R32 GTSt works the same way, but it is entirely possible that there is a ~20-25% difference in the cable speed required to work the 2 different speedos, because the 2 different boxes (32GTSt and 32GTR) are not in any way related to each other.

So, I reckon that your easiest test (not that it's all that easy) is to try a GTSt cluster in the car and see if the speedo reads correct (or actually, maybe about 4.5% wrong rather than 25%).

I don't know anyone else who has transplanted a GTR cluster into a GTSt to know if there is that big a difference in the cable speed required for a given road speed, but I'd be willing to believe that there is and that many people who have done it either haven't realised or just live with it!

FWIW, if you do the Navara sender based mech speedo conversion (like I did in my R32) for the RB25 turbo box, then you can get nearly zero% speedo error. Mine is accurate to within 1km/h across the whole normal range (say up to about 130 km/h).

[CtrldKilla]

BUT....and here's the big but.....The GTR speedo expects to be turned at a particular speed by its speedo cable. That speed is dependent on the number of gears on the drive in the box and the end of the cable. The R32 GTSt works the same way, but it is entirely possible that there is a ~20-25% difference in the cable speed required to work the 2 different speedos, because the 2 different boxes (32GTSt and 32GTR) are not in any way related to each other.

Awesome, this was basically my train of thought however I didn't know enough to make any real conclusions. It's good to know that I should be able to rule out the diff/box.

I will see if I can get my hand on a GTS-T cluster, I'd rather a GTS-T cluster anyway, while the red and silver dials are nice, the "Front Torque" dial is useless and just makes the car seem "ricey".

When I get my hands on one I will post the results up here and let you know.

Cheers!

[CtrldKilla]

A dude on a buy and sell page just offered me a cluster for $80, are they worth that much?

Edited February 4, 2015 by CtrldKilla

[jiffo]

You need to do more research.

I'd say the drive pinion has the wrong tooth count, something for you to check.

You can't just buy any GTSt instrument cluster as there's at least 2-variations through the years.

You'll need to verify your model car and buy the one that suits.

25kph wrong is a biggie.

Are you sure the previous owner didn't remove the needles and replace them incorrectly. (the white faced dial syndrome)