Speedo Corrector

[Blk33]

Speedo Corrector

Last year I picked up a 1996 R33 GTR direct off the boat from Japan through a dealer in Melbourne. To my surprise when I was driving down the freeway towards home sitting on the maximum legal speed of 100KMs per hour, I was being passed by everyone on the freeway. This is not what I had planned for my new GTR! How could this be? My initial guess was that the speedo was reading fast, but what was is due to? The GTR has a 320Km Nismo speedo, perhaps this could be a problem?

Testing

A few weeks later I mounted my Tomtom One GPS and went for a drive. Sure enough at an indicated 100Km on the GTR speedo the GTR was only doing about 94Km as indicated on the GPS. I checked the linearity of the speedo at an indicated 50Km and the GPS read 47Km – linearity was constant. Why such a 6% error?

The tyres fitted to the car from Japan were a 255 x 40 x 17. The original tyres fitted to the R33 GTR were a 245 x 45 x 17. I check a great web site ‘the Car Maintenance Bibles’ http://www.carbibles.com/. In the Wheel and Tyre section on page 2, there is a tire size calculator. I keyed the two tyres sizes into calculator and it shows the fitted 255 x 45 tyres are 52mm shorter in circumference than the 245 x 45 tyres, which effectively over speeds the speedo by 2.5%. Thus at an indicated 100Km the actual speed would by 2.5% less, or 97.5Km. I guess the other 2% or 3% would be an error built into the speedo at manufacture.

In the state of Victoria the constabulary like to ensure motorists do not travel above the speed limit and are keen to enforce these limits with heavy fines for those that exceed them. Because of this I like to know exactly what speed I am driving at and a 6% error is too large.

Solution

I remember I had bought a Silicon Chip Speedo Corrector to fit in my son’s Clubsport a few years ago. He had changed the diff from a 3.45 to a 4.1 and consequently the speedo read too fast. Any way he sold the car before I fitted it, so I dug it out.

The device is a Silicon Chip kit - number KC5435 http://autospeed.com/cms/A_108415/article.html

I had purchased the kit through Jaycar http://www.jaycar.com.au/ My kit was an older version that didn’t have an interface for Nissans so I had to modify the circuit board for this. The circuit board has two rotary switches that switch in a 1% increase or decrease in pulse width to re-calibrate your speedo. One rotary switch covers 10% steps; the other rotary switch covers 1% steps.

1. Finished Circuit Board – this is the earlier version and is functionally the same as the newer boards - the new boards have more options

Interfacing to the R33 Speedo

I decided the easiest place to break the incoming speed signal from the gear box was at the rear of the speedo, direct it into the speedo corrector, make the adjustments for the error, and then feed the corrected signal back into the speedo. The first step was to locate the signals at the back of the speedo.

2. Rear of Speedo showing required pins

I have indicated on the picture the 3 signals that are required for the Speedo Corrector. +12 volts and ground to power the Speedo Corrector and the incoming signal from the speed sensor on the gear box. I soldered 3 wires directly onto the speedo circuit board and used the isolating washer and screw to feed the modified signal back to the speedo.

3. Isolating Washer and Screw – This is from a transistor heat sink kit

4. R33 Speedo circuit showing the attached 4 wires

I soldered 3 wires to the circuit board and attached the 4th wire to the isolated washer and screwed it back into the speedo. This effectively breaks the incoming signal from the gear box, diverts it to the Speedo Corrector and feeds the modified signal back to the speedo.

5. The finished board with cable

Above shows the cable, I also used a plug in case I had to disconnect the Speedo Corrector. I mounted the board in a small plastic case, wrapped the box in foam, and shoved it up under the dash board.

Testing

The Speedo Corrector worked like a dream! I mounted up the GPS again, left the rotary switches at zero and went for drive down the freeway to confirm all was working and no additional errors had been introduced. I then dialled in a 5% increase and then checked the speedo at 120Km and 60Km. At 120Km on the GPS the GTR speedo read about 122Km, and at 60Km on the GPS the speedo read about 61Km. Job was now

[some_cs_student]

The pics are missing?

The UK version has them:

http://www.gtr.co.uk/forum/111435-speedo-c...ed-r33-gtr.html

Excellent write up, I'm off by at least 7km/h at 100km/h last time I went through one of the highway tests, I'm considering doing this mod sometime...

[HtwCHACtkh]

This is what I found interesting..

You don't need any extra electronics to calibrate your speedo. There are a set of tabs on the rear of the R33 speedo that can increase or decrease or increase the displayed speed. You can even modify a kph speedo to correctly display the speed in mph without altering the signal to attessa etc. This is confirmed by monitoring my PFC. You can even adjust the odometer so that it also clocks up in miles rather than kms. AFAIK the nismo, UK and standard GTR/GTS speedo, PCB all use identical PCBs and it is just a matter of setting the links up correctly and maybe adjusting a resistor (just for the odometer reading).

Can anyone confirm this for an R33 GTS-T (And other models for everyone else)? And know how it works?

Having my speedo go haywire is the thing which is stopping me from changing my wheels.

[gotRICE?]

I guess its always good to be doing the speed indicated, but would it not be easier (especially if its such a small error) to figure out via GPS what speed your speedo sits on @ 100km/h and just travel with that?

Aussie is tough! NZ give you a 10% speedo manufacturing error @ 100km/h.

[some_cs_student]

This is what I found interesting..

Can anyone confirm this for an R33 GTS-T (And other models for everyone else)? And know how it works?

Having my speedo go haywire is the thing which is stopping me from changing my wheels.

I had a go at it here, I was about 7km/h out to begin with, made it worse than got better through trial and error, I provide no guarantees it will work for anyone else.

http://www.skylinesaustralia.com/forums/R3...02#entry4570002

Please read through the posts that I link to, there is a 10page article on how to go from km/h to mp/h, it provides a lot more detail than what I posted but what they did didn't work for me

[c-gtst]

I had a go at it here, I was about 7km/h out to begin with, made it worse than got better through trial and error, I provide no guarantees it will work for anyone else.

http://www.skylinesaustralia.com/forums/R3...02#entry4570002

Please read through the posts that I link to, there is a 10page article on how to go from km/h to mp/h, it provides a lot more detail than what I posted but what they did didn't work for me

might have to give this a go because i think i might be out a bit aswell!