Speedometer Calibration

Now that the car is pretty much complete and ready for testing, I thought I had better calibrate my Smiths Speedometer.

I bought the Caerbont Automotive Instruments Smiths Electronic Speedo (200mph anti-clockwise).

To be able to calibrate the speedo, you need to follow the instructions very carefully. (Online Download)

The first piece of information requried is the rolling circumference of the rear tyre.  I stuck on a piece of masking tape onto the tyre and drew on a straight line.  I then placed masking tape on the floor with a similar straight line.  From the starting point of both lines touching, I rolled the tyre across the floor and when it went full circle, you guessed it, I placed another piece of masking tape down with another line.  Measured between the lines I got a measurement of 2087mm.  This is for the 275/40 R18 Michelin Pilot Super Sports.

Since my speedo is in MPH, the instructions request "Wheel revs per mile = 63360 divided by the distance travelled in inches".  

To convert from mm to inches you need to divide 2087 by 25.4.  This gives the answer in inches (82.16535433)

Therefore the Wheel revs per mile for my car is 63360 / 82.16535433 = 771.1279348 (give or take a few decimal points!)

We can now figure out the calibration number with the following calculation "Calibration number = (wheel revs per mile/km) x (diff ratio) x (number of magnets or bolts)"

The diff ratio for my Dana Spicer Hydratrak is 3.07:1

The VSS for the T56 Magnum has 17 pulses per revolution (That's what most people seem to confirm on the online forums!)

So, finally, the Calibration number sum of the speedo is:

771.1279348 x 3.07 x 17 = 40245.16692

The instructions state "the calibration number you have calculated should be rounded up to the nearest whole number" so my final Calibration Number is 40246.

To enter this data into the speedo itself,  as per the instructions:

"The calibration mode is selected by switching on the ignition while simultaneously pressing the trip reset button. The pointer will travel to full scale and return to zero. If the button is released before the pointer returns to zero, the manual setting procedure will be selected and the LCD (ODO.) will read ‘SET PPU’. If the button is released after the pointer returns to zero the ‘drive to set’ mode is selected and the LCD will show ‘DTS PPU.’  At this point, pressing the reset button momentarily will toggle between ‘SET PPU’ and DTS PPU’.  

Manually Inputting The PPU Number

Set the LCD to display ‘SET PPU’ as described above.  Press the reset button for 2-3 seconds, the LCD will show the calibration number currently set. Each digit within the calibration number will flash in turn for approximately 2 seconds. When a digit is flashing, each depression of the reset button will increment the digit by one. Once the last (right-most) digit has been set/reset, the whole number will flash. Press the reset button and the LCD will display ‘DONE.’ After 3 seconds the LCD will display ‘SET PPU,’ the setting is now complete. Switch off the ignition."

It is fairly easy to setup once you know all your figures - as long as you remember to press the reset button when all the digit flashes to ensure the number you have put in is stored...  When I double checked that I had put the correct number in, I found that the default number "008380" was still there.  I re-did it and remembered to press the reset button when all digits were flashing.  I have since re-checked, and my number has been stored.  I will test it properly once on the road compared against the sat nav.