How fast am I going?

For a long, long time, I have known that the speedometer on the bike reads high, nine percent high in fact.

This hasn't been a real problem so far, as I know where 100 kph and 110 kph are on the speedometer.

Lately I have had to know lower speeds a bit more accurately, as cars are becoming more and more aggressive/competitive for space on the road. Going a bit slower than the slated speed limit causes some major irritaion amongst car drivers.

The only real solution was to fit a push-bike computer to the bike.

Issues were:

• Modification.
  The cable between the sender and the display unit is usually too short, necessitating a bit of cutting and soldering of an extra bit of cable.
  
  
• Determining the right tyre circumferance to show an accurate speed.
  In the past I have measured and then finally used the distance between road signs indicating distances to a town. Another method is to use those overhead gantries that give you a speeed check This has not been of much use lately as they fail to see me and I have to travel at the same speed as a car and use their displayed speed as a measure.
  This time I used a GPS unit and fiddled with the circumference values until I settled on 1970 mm.
  
  
• Buying the right computer.
  My first attempt was a real cheapy, $20. After fitting it I discovered that the indicated speed would not exceed 99.9 kph. Close to what I wanted, but not just quite right.
  
  
• Fitting the magnets.
  I had three attempts at this and the two issues are having a strong magnet and using the correct glue:
  • I used Araldite the first time and achieved about 40 km before it flew off.
  • I used Super Glue the second time and achieved about the same result.
  • I used more Super Glue this time anchoring the magnet at the base and the sides. Hasn't fallen off so far.
    I purchased a pack of about ten circular magnets from Bunnings after the first attempt. I have to use two glued together to obtain the desired height.
    
    
• Hardware failure.
  I had two hardware failures:
  • My soldering wasn't quite so good initially and one of the wires separated.
  • The reed relay sender failed requiring a replacement. I had used the original sender from years ago as it was physically more robust. Maybe it just got tired?
    

After all of that I have confirmed that the speedometer does indeed read nine percent high.

I wonder if there is a 'gain' control somewhere inside the speedometer so I can calibrate it?

The Result:

My speed has increased somewhat and I travel with the traffic now instead of being passed all the time. I think that is good for my health.

Foot Note:

Why is it that the Odometer only reads two percent high while the Speedometer reads nine percent high? They both use the same pulses from the sender located in the differential.

Quirky fact:

In the brochure for the bike, fuel consumption is quoted at 91 kph. I've always found that a rather quirky speed. That equates to 100 kph at the Speedometer. Is there something I should know?