Hey guys -

Did some figuring out today and made a cool discovery! Time for a good writeup .

OK, as some of you may remember, I put an RX-8 4.44 rear end gear in my FD -

https://www.rx7club.com/3rd-generati...-cover-997318/

I've been running that setup for a while now, VERY happy with it. The big thing I haven't been happy with, though, is the speedometer error. It's close to 10% or so, which doesn't sound like much, but when the speedo says you're doing 90, you're actually doing 80. I guess it can keep you out of a speeding ticket, but it's REALLY annoying.

So, I've been looking into how to fix this problem. There are devices that can modify a digital speedometer signal to compensate for R&P changes, tire size changes, etc. Mighty Car Mods just put out a video this week on a digital speedometer corrector that got me thinking about this -


(BTW, if you guys haven't started watching MCM, do so! Really neat web series).

While a digital corrector is one good way to do it, I really wondered if there was some other adjustment on the speedo itself. 240SX's have a number of traces that can be cut or wired to correct the speedo, for example, so I decided to dig in.

Pulled my instrument cluster out and removed the outer covers. First off, I'm running my original, stock, US speedometer - it's the original one to the car. Along with that is a '99 6 o'clock tach and a JDM oil pressure gauge.



You can (obviously) see the speedo on the right. If we get over to the right side of the cluster and take a careful look, we can see a small blue poteniometer (pot) -



The pic above I'm pointing at it with a screwdriver so you can see the relative location. Close up pic of the pot -



OK, so we've got an adjustment pot. Fun! Now what to do...

First off, I drilled a hole in the white plastic body of the instrument cluster for a straight shot at the pot with a screwdriver. If you look in the second pic, you can see the hole. I drilled a few holes next to each other to make a larger hole, that made it possible to not only have a screwdriver in there to adjust, but to also look in at the pot and watch it as I adjust it.

The pot has a WIDE range of adjustment. Here's the rules of adjusting the pot -

- It is very sensitive in that a small turn will easily change 3-5 MPH.
- Turning CLOCKWISE will REDUCE the speedo reading.
- Turning COUNTERCLOCKWISE will INCREASE the speedo reading.

OK, so we now know how to adjust it. How do we know how MUCH to adjust it?

You need a speed reference. The good thing is this is easy. Most car GPS's will give a very accurate speed on their display. I found even better than that to get a speed app for my phone. I have an Iphone 4, I got an app called "SpeedBox Lite". It was free, it displayed the realtime speed in MPH, KPH, or even knots, was easy to read, and kept the phone from going to sleep/locking while it was active. This worked PERFECTLY for me. I'm sure there's an Android equivalent app, just search around.

OK, armed with that and a freshly-drilled hole in the side of the cluster, I put the cluster "glass" cover back on to protect the gauges (I didn't put it all back in the full instrument cluster shroud) and plugged in the 4 connectors to the back. I just set the cluster in place and brought a #2 Phillips screwdriver to adjust the pot.

To start off with, I cranked it all the way counterclockwise and went for a drive. Going down a residential road I saw that my signal was WAY too high - the speedo was showing 60mph and I was MAYBE doing 20-25! So, that's the wrong way. Pulled over, shut off the car, cranked it the other way. Now it was showing barely any speed when driving - like 5 mph or something. Good.

So, I kept doing iterations of that - see what my error was like compared to the GPS, pull over, shut off car, make adjustment, start car, and test again. You may be able to adjust it while the car is running, but I didn't want to try that.

I eventually got it to within 3 mph at 30mph. Getting close. Now, tiny adjustments are key - like BARELY bumping the screwdriver into a turn. Once it seemed pretty close at 30, I went to 40, got that close, then to 50 and got it dead-on. At that point you should be within a good margin, if it's accurate at 50, you're going to be accurate up higher, but you could keep tweaking if you so desire.

Now, here's the biggie, I think this might be useable to re-calibrate a KPH speedo to read in MPH. So, it would say "50 KPH" but you'd be travelling at 50 MPH. That is HUGE. If that works, you could buy a JDM speedometer (of which there are MANY more that are in good condition!), calibrate it, and have an accurate speedo!

It makes sense that Mazda would just make one speedo mechanism, put the appropriate faceplate on, then have a factory worker calibrate it to that face. That makes MUCH more sense than entirely different speedo "guts" for each market. This could also explain how there are many JDM aftermarket speedos that read up to 300 KPH - they just bought the same mechanism, got a faceplate made, and calibrated it.

Most likely next weekend I'll have a chance to test that theory - a buddy of mine has a bad speedo and has been running a JDM speedo (reading in KPH). So we'll see!

Anyhow, this is VERY exciting and a VERY simple and easy fix!

Dale

 

(rx7club)--> <--(Dale)
Thanks for all this research!

 

Very nice info!!! This would allow for someone to do a whole build and calibrate for every change all at once. Ie wheel and tire combos and rear end ratios.

Now... the real question after that is how do you take the needles off and put them back on and still be calibrated for the odometer?

 

I've pulled tach needles before, it's VERY tricky. The big trick with pulling needles is to detach the gauge face so it can rotate. Rotate it so the needle is pointing somewhere at rest and not on the stop on the gauge face, then mark that spot. Pull the needle, and when you re-install it, point it at that same spot.

BTW, if you're calibrating just for a small change (wheel and tire size), you need VERY small adjustments to the pot to make a difference. I can't stress that enough. If you go wild, you'll be doing a LOT of driving around trying to get your speedo halfway right again .

Dale

 

another idea to add to this for those who want something different. have a shop make a new faceplate for the OEM speedo like say speedhut or black cat customs.

once calibrated you could have a 200, 220 or 250mph speedo. not like everyone would use that but i envision some cars may be geared that high with auto gearsets.

 

^Yep. You could also get the JDM 300+ KPH speedos that the JDM tuners sold and make that MPH. You'd have no probs with it reading too low .

Salt flats, anyone?

Dale

 

This MUST be a sticky and linked to the FAQ!!!

 

i'd just prefer it read the proper units unless you just etch out KMH and label it MPH.

wish the FCs had something similar, i'm annoyed by the fact that my speedo is off by 2mph at 75.

 

Crap, I edited the article, the pics disappeared!

Pic of cluster -



Pic of side of cluster with screwdriver pointing to pot -



Closeup of pot -



Maybe a mod could fix this?

 

they're still in the original post.

 

Great write up Dale!!

 

Dale Clark strikes again....

big THANKS for the research and sharing...

 

Did it today on my friend Jimmy's FD who also switched to a 4.44 rear end (and has aftermarket wheels). With 2 iterations we were able to get it dead on. Hope next weekend to try a JDM speedo!

On my car I've also driven by those radar "your speed is" things, and they were dead on as well (they were off before).

Dale

 

Great find, Thanks

 

So what sources of faces are out there?

 

Hot damn Dale.....

This is awesome.... My factory gauge with factory sized tires is out 10kph at 100kph, (6mph at 60mph)


Now I can fix it, and not rely on my radar detector/GPS for speed readings....


J.

 

Great info, thanks Dale!

 

Great find! Interested to see how you get on with the jdm speedo.

Unrelated...But did you change the film on you '99 tacho to make it backlit amber?

 

I think with the factory error plus larger wheels and tires you can easily get to a not-acceptable level of error. Now it's easy to fix .

My '99 tach does light up green. I actually dig it, the other gauges are red, the tach is green, and my 2 Greddy gauges (boost and water temp) are green. Works for me.

As far as I know there are no other faces by themselves on the market that would change the reading. But, there are TONS of Japanese speedos out there that read to 280kph or 300+ KPH. You will pay a premium for them, however.

Dale

Dale

 

thanks for this!!

 

Any info on making the 180kph speedo read mph?

 

That, or if you had an old non working cluster...just switch out the faces/film that has the markings on it.

 

I'm glad to know you Dale!! You want to try this on KMH cluster?? I got couple

 

Does the odometer get its information by extrapolating time vs speed from the Speedo signal?

 

Dale,
This is awesome. I hope it works for the kph speedo. Time to pull my cluster I guess!

Regards,
Crispy