Installation of Yellow Box Speedometer Recalibrator

The Yellow Box (YB) speedometer correction recalibrator corrects for built-in manufacturer speedometer error as well as the additional error caused by the installation of smaller or larger sprockets. Built-in error has been estimated by IowaZ at an average of about 5% (see Here) and by CycleStatstm at 4%. the additional error (in the same direction, reading high) caused by the smaller 15-tooth sprocket is 6.67%. Total estimated initial correction is 11.67%. The YB (fine ratios model) allows a range of +/- 28% in 0.25% steps. I initially set it for 11.5%.

This shows the YB switches set for the 11.5% correction.

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The Yellow Box comes with:

  • The Yellow Box (Bike Model, Fine Ratios)
  • Wiring Harness
  • Jumper Plug
  • Instructions

The purpose of the Jumper Plug is to permit a return to stock wiring conditions in the event the YB is removed. Plug the end of the harness into the jumper plug instead of the YB for stock speedometer operation. It only fits one way.

The YB also corrects the odometer. Since the sprocket installation will cause a 6.67% high reading, the odometer will show 6.67% more miles than actual. This device corrects this situation.

The Yellow Box may be ordered directly from Black Robotics in New South Wales, Australia from its website:

The price is US$100 including shipping. Turnaround is about 10 days.

An alternate product, the SpeedoHealer, is described here.


Remove seat and right side cover.

A convenient place to make the connection is next to the speed sensor coupler under the right side cover. The Yellow/Blue wire is the +12V source. The Black/Blue is the ground wire. The White wire is the speed signal. Cut the wires about in the center and strip the ends (16 gauge wire).

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Wrap the harness wires in electrical tape. Cut about an 18 inch length of harness and strip the ends (16 gauge wire). You may have to adjust this length after the harness is in place.

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Place the YB under the seat and route the harness down to the splicing area. You may select an alternate location but make sure the box or wires aren't pinched or crushed when replacing the seat or side cover.

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Make the connections as follows:

  • Yellow/Blue Wire from Speed Sensor + Yellow/Blue Wire to Speedometer + Red Wire to YB (I soldered a short length of Red Wire onto end of Yellow/Blue Wire from Speed Sensor)
  • Black/Blue Wire from Speed Sensor + Black/Blue Wire to Speedometer + Black Wire to YB (I soldered a short length of Black Wire onto end of Black/Blue Wire from Speed Sensor)
  • White Wire from Speed Sensor + White Wire to YB (I soldered a short length of Green Wire onto end of White Wire from Speed Sensor)
  • Green Wire from YB + White Wire to Speedometer
I soldered those extra short lengths of wire to make the connections easier to reach without placing a strain on the wires. My color selection for those extra lengths was, unfortunately, arbitrary.

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I believe there is an error on the Factory wiring diagram with respect to the colors for the wires on either side of the speed sensor connector. The colors shown on the diagram appear to be reversed. That is, the diagram showns an Orange/Red wire coming from the speed sensor changing into a Yellow/Blue wire after the connector. It also shows a Pink wire coming from the speed sensor changing into a white wire after the connector. Lastly, it shows a Black/White wire coming from the speed sensor changing into a Black/Blue wire after the connector. I contend these colors are reversed with respect to which side of the speed sensor connector they are on.

Finish off with electrical tape. It is also a good idea to cover the box and connector with some sort of plastic bag or wrap to protect it from dirt and moisture. Finally, replace the right side cover and seat.

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If I had this to do over again and had more patience I would track down a three-wire connector like the speed sensor coupler and manufacture a spliced connecting piece to insert between the ends of the existing speed sensor coupler.


There are a number of test modes to verify the operation of the unit. I recommend trying these prior to soldering or finalizing any connections. The modes are set with the switches and read by following the resultant sequence of short or long flashes of the LED light. The red LED light is under the yellow epoxy and shines through. It is not an external LED! I initially had difficulty in two areas.

First, I followed the wiring diagram and obtained no speedometer reading. Upon reversal of the leads defining to/from the speed sensor and speedometer, I got a reading. This is why I believe there is an error on the wiring diagram. Also, a closer examination of the wiring on the bike confirmed this.

Second, the unit I received had a problem with switch number 3. No matter what position it was actually in, the flashing light sequence used to display the sensed switch positions indicated it was always in the DOWN or OFF position. In this condition I was unable to try the various test modes all requiring switch number 3 to be in the ON or UP position. Advice provided by Roman Black from Black Robotics (and included in the supplied instructions) was to spray some rubbing alcohol on the switch and to exercise it ON and OFF about 20 times. I added some blasts of compressed air. This fixed the problem temporarily which was probably a bit of dirt or dust that had found its way between the gold switch contacts. Unfortunately, the problem returned and I was offered and received a new unit in exchange.

I have been advised by a pro audio expert that rubbing alcohol leaves potentially damaging residues as it contains lubricants for the skin. Isopropyl 99% alcohol is much better but the stuff you should use is a professional cleaner such as that from Caig Laboratories.

The output test mode does not produce a reading on the speedometer, presumably because the speedometer starts at just below 10 mph and the test signal of 100Hz produces a reading of less than that.

Black Robotics provides excellent email support. My questions and problems were addressed with prompt and courteous responses. Keep in mind the time zone differences with Australia.


I went for a ride to compare displayed speeds versus rpm in 1st, 2nd, and 3rd gear and compared the readings with theoretical speeds from the following table and plot. It's difficult to read a precise speed from the speedometer but you can get a fair approximation (+/- 2 mph). I also could have also done this (carefully) with the bike on the centerstand, in gear, and in a well-ventilated area. Doing this for too long however can cause the engine to overheat.

Theoretical Speed in Gears
Engine RPM 1st Gear 2nd Gear 3rd Gear

The results at the speeds I conducted the test with an 11.5% correction did not match. The speedometer read too low. In increments I reduced the percentage correction until a good match was found. This final number was 6.75%. Comparison of speeds shown with a GPS confirm this result. Now, its very difficult to read my particular GPS while moving given its averaging time and my admittedly poor ability to hold a constant speed for the required time. But, the match is good and the engine speeds compare well. Placing the YB in the no correction mode and comparing this with GPS displayed speeds did indicate a high reading in the rough area of 7%. Summarizing:

  • I could find no stock speedometer error component at the speeds investigated.
  • A correction value of 6.75%, closely matching the 6.67% mathematical error introduced by the smaller front sprocket, is the minimum recommended.
  • If you wish to include a stock speedometer correction for higher speeds you can correct anywhere in the region of about 9-11%. You won't be too far off at low speeds.
  • I am somewhat dissatisfied by these results because others have found stock speedometer errors and I believe that there probably is one.
  • More precise tests should be conducted, perhaps using a radar gun type speed measurement on a controlled street.

Personally, I decided to split the difference between 6.75% and 11.5% and settled on 9.25%. With this correction my speedometer reads low by about 1 mph at low speeds and high by about 2 mph at high speeds.


Speedometer error tests conducted by Rick Chapman indicate the following results, all with the speedometer reading high. These are all stock speedometer error values independent of any non-standard sprocket induced errors.

  • 62 mph - 6.2% error
  • 121 mph - 9.0% error
  • 128 mph - 10.5% error

This confirms other results that found that the speedometer error is not a fixed percentage but a percentage that increases with speed. This presents a problem for devices like the Yellow Box which correct by a fixed percentage.

Unlike the speedometer, the odometer is very accurate. Government regulations undoubtedly require this to be the case. So, if you correct the speedometer for stock errors you will be understating the actual mileage of your bike as reported by the odometer. Correcting the speedometer for sprocket induced errors only will preserve the accuracy of the odometer. Correcting the speedometer for a combination of stock error plus sprocket error will result in the odometer understating the mileage by the amount of the stock speedometer error only.

Addendum II:

With the installation of the larger 47-tooth rear sprocket I adjusted the Yellow Box to correct for the 14% known error introduced by the two sprockets and added back in the 2.5% arbitrary speedometer error correction. The total correction is now set at 16.5%

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Addendum III:

I replaced the Yellow Box with the SpeedoHealer when a sample of Version 3.0 of that unit was provided for evaluation.

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Last Updated: 04-30-2005

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