Why is My Motorcycle Speedometer Wrong? | Ask the Geek

Why speedometers lie, and how to correct them

motorcycle speedometer, inaccurate speed readings, how to fix motorcycle speedometer
Seems pretty accurate right now…Courtesy of Honda

Speedometer Error
On every bike I've owned, the speedometer has read much higher than actual road speed, whereas my cars have always been almost spot-on. Why is this? I want to fix the speedometer on my Honda CBR1000RR using something like a Speedo­Healer. I know those devices tap into the speed sensor. Should I be worried that it will affect how the ABS works on my bike?

Shawn Baker
Stewartstown, PA

On testbikes we typically see an error of a few percent in the speedometer reading—at times up to 10 percent and always on the high side. This fudge factor can most likely be traced to ECE Regulation No. 39, which is a 14-page document detailing speedometer accuracy for vehicles sold in EU countries. According to this law, a speedometer can read high by as much as 10 percent plus 4 km/h at a specified test speed, but under no circumstances can the speedometer read low.

Speedometer accuracy is very dependent on tire diameter, which is affected significantly on a motorcycle by pressure, load, and temperature. These values are strictly defined in the regulation's testing procedure, and the speedometer calibration must take these potential alterations into account. In addition, the manufacturers must consider the owner changing to non-standard tires, which may have a different diameter than the OEM fitment. In our last tire test, diameter of the seven rear tires tested varied by 2 percent, which directly affects the speedometer by the same amount. This also accounts for some of the discrepancy. Even with all these variables, however, the manufacturers do seem to be erring on the side of caution so the speedometer doesn't read low.

Most current motorcycle speedometers use a sensor triggered by the teeth of a transmission gear or a similar dedicated ring or by the slots in an ABS wheel ring. These sensors send a square-wave signal to the ECU, which calculates speed from the frequency of the signal. As mentioned, tire diameter does affect the speedometer, but if the sensor is in the transmission, changing gearing can also affect the reading. Aftermarket devices like the SpeedoHealer tap into the sensor wire and speed up or slow down the frequency of the signal, changing the speedometer reading accordingly to recalibrate it for any error or a gearing or tire change.

On most bikes equipped with ABS, the speedometer works from the same signals as the ABS to measure speed; on these bikes, I would not recommend using any device that taps into the sensor or alters the readings from the ABS rings, as that could affect your ABS, and this would not be something you want to test or find out right when you need it the most. There are a couple of workarounds here: If the ECU sends a square-wave signal to the dash specifically for the speedometer, a device could be inserted here as opposed to in the sensor wires. And some bikes can have the speedometer recalibrated by an ECU re-flash.

Your CBR1000RR C-ABS is a bit of an anomaly, however, as it retains the countershaft speed sensor of the standard model to measure speed. Here you could install a SpeedoHealer or similar device with no worries of affecting the bike's ABS.

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