K bike speedometers usually run about 10% fast from the factory. However, it is fairly easy to recalibrate one yourself.

Many soldering irons emit a magnetic field at a frequency of 60 Hz. (In the US.) This is the frequency of normal household 120V alternating current.  If the magnetic field put out by your soldering iron is strong enough (some are, some aren't) then you can use that field to put a 60 Hz signal through the speedometer sensor from your K bike's final drive is on order to calibrate the speedometer.

Determine V₁: Remove the speedometer sensor from your final drive, turn the ignition switch to the on position, plug your soldering iron in and hold it close to the speedometer sensor.  Your speedometer needle should now read somewhere in the middle to high 40s MPH.  Record or remember that speedometer reading. That speed is V₁.

Determine V₂: Re-install the speedometer sensor and go far a ride with a GPS.  Ride your bike so that the speedometer reading is a constant V₁.  Look at the speed on your GPS. That speed is V₂, what a calibrated speedometer should read when the speedometer sensor is putting out 60 Hz.

Remove instrument cluster: Remove the four 5mm Allen bolts on the back of the instrument cluster that hold the instrument cluster in it's bracket. Then, depending on the model year of your K bike, there is either a Phillips screw or 3mm Allen bolt that holds the wiring harness connector to the back of the cluster.

Remove instrument cluster back plate: Place the instrument cluster face down on a towel or something to prevent scratching it.  Depending upon the year, there are either 7 or 9 Phillips screws around the perimeter securing the back plate.

Remove instrument cluster internals: There are six screws that hold the internals in the instrument cluster - four at the middle bottom and one on each side. Remove those six screws, turn the cluster over and carefully shake the internals out - pull out the trip odometer knob as far as it lets you while doing this - there's a notch in the side of the speedometer face plate which lets it by the trip odometer rod.

Warning: Be very careful with the instruments once removed.  Try not to get fingerprints on the face plates and be very careful with the speedo and tach needles - they will fall off if you look at them crossways.

Loosen the speedometer from the assembly:  There is a very small black screw in the white plastic in the lower outer corner. That will loosen the speedometer subassembly from the rest enough for you to gain access to the potentiometer which controls the speedometer.

 

Locate calibration potentiometer: The arrow below shows the potentiometer that controls speedometer calibration.

Adjust the potentiometer: Use a small screwdriver or knife tip  to turn the potentiometer counterclockwise about 45 degrees which will get the calibration pretty close.

Test and adjust calibration: Remove the speedometer sensor from the final drive again.  Hook the instrument cluster back up to the bike, turn the bike on and place the soldering iron near the speedometer sensor again.  The speedometer should now read close to what you found was your true speed(V₂) at 60 Hz.  Perform minor adjustments to the potentiometer as necessary until you are satisfied that the speedometer is properly recalibrated to read V₂ at 60 Hz..

Warning: If, when testing, you misalign the connector pins on the instrument cluster when hooking it up you can fry your $75 Bulb Monitor Unit in the relay box.

Reassemble:  Reassembly is simply the reverse of disassembly.  Be sure to remember to pull the trip odometer out while sliding the internals back into the housing.

 

Alternate Calibration Methods

Karamba:

There is a Windows PC program, Karamba, which generates an audio output signal that can be used to calibrate K bike speedometers. Click here to download Karamba.zip.

Instructions for it can be found at this link.

Here's an English translation of what appears on the screen:

Caveat: I have used this before and found that some laptops and netbooks, in order to conserve battery power, do not put out a strong enough audio signal and the result is that the speedometer reading when using Karamba will be too low.

Sine Wave Generator Software:

I use the sine wave tone generator of the freeware Audacity and the following table that I put together for the various rear tire sizes:

 K.Bike.Speedo.Cal.pdf

And, since I like to do this on the workbench, I built this wiring setup to simplify things:

© 2013 Drake Smith - Please do not use or reproduce this elsewhere.  Feel free to link to it though.