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.

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.

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.

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

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.

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 done.