Adjusting a FD speedo - different diff ratio
 

Hi guys/gals,

I have asked this questions a while ago with no luck in another section so thought I would try my luck here.

Old thread: https://www.rx7club.com/showthread.p...98#post9792098

I have a FD with a mazdaspeed diff with 4.8 ratio and this has put my speedo out by 20km/h at 100km/h (reads 120 at 100) - sorry for the metric.

I have done all the diggin I can about how the speed sensor on our cars work and cant seem to find anything. Most cars has one signal wire that goes to the speedo that can have a simple kit installed to modify the signal and adjust the output, but these kits refuse to work on the FD. From the diagram in the above link, the sensor has two wires that run from the sensor to the gauge cluster, then who knows what happens between there and the speedo gauge itself.

Has anyone managed to correct their speedo for this sort of thing? or does anyone know when I can find any info on it?

It is my understanding that the speedometer correction units worked. If you don't want to spend all the money on that you can manually adjust your speedometer by opening up the gauge cluster and adjusting a potentiometer I believe.

I think there have been a lot of members that have been successful with this.
http://www.dakotadigital.com/index.c...rod/prd126.htm

I have a very similar kit that I cannot get to work.

Is there any info out there of which of the two wires to intercept as the signal wire? I have tried both with no luck.

I will open up the cluster and have a look for any adjustment in there though. Thanks.

More information on this? Would be nice to adjust kph cluster to mph this way opposed to swapping speedo's.

I don't think this would work to convert from kph to mph. I would think the scaling would be different and the pot would just change the "zero" of the gauge. I'm not sure though.

Here is some good information to understand what I am talking about.
http://www.zeroyon.com/index/content/view/20/47

The base resistance may not need to be adjusted on the RX-7 speedometer PCB.

In the pictures below you can see the blue potentiometer on my JDM speedometer I have.

Thats just what I am after. Thanks ObliqueFD, ill give that a try.

hmmmm on another note, I wonder if I could half the reisitance and make the speedo read half the actual speed...ie 50 on the gauge = 100. would give a 360k speedo....not that I would need it haha.