RX3 Speedometer and odometer calibration

It's no secret that the speedometers on most bikes and cars are optimistic, reporting a speed that's higher then the true speed. This is obviously deliberate and the reason maybe twofold. First, in Europe, there's a regulation that no speedometer is allowed to read a speed that's lower then the true speed, so to make sure that regulation is complied with manufactures make sure to make the speed read higher. While no such regulation apparently exists in the US, fear of law suits over speeding tickets probably results in manufacturers making really sure that they can't be blamed if you get a ticket for 60mph in a 55mph zone. I could not find any regulations related to odometer accuracy.

So how far off is the RX3? Well, to test it I installed a true reading odometer/speedometer on the bike that could be adjusted for wheel size. I measured the circumference of the front tire at 1.93m by rolling it along the ground and measuring distance and number of revolutions. I also carried three different GPS units, a handheld Garmin etrek, The GPS on my Android smartphone and one i-gotU GPS data logger. All three could measure and record speed and distance. So that's 4 independent ways of measuring speed and distance in addition to the RX3's own speedometer and trip meter odometer. In addition to that I used Google maps to measure the distance on the route I rode. It seemed accurate too since it agreed well with the GPS measurements.

While the three GPS units, Google and the calibrated odometer/speedometer didn't agree exactly, they were all pretty close (within 1%) - and significantly different from the RX3's own instruments. The bottom line after averaging the readings was:

Since both the speedometer and odometer are driven by the same signal - a series of pulses from the speed sensor on the front wheel, it's not possible to recalibrate both the speedometer and odometer by modifying the pulse stream. Commercial devices exist that look at the pulse stream and increase or decrease the frequency to change the speedometer/odometer readings. If you decrease the pulse frequency by 6%, the odometer will read right but the speedometer will be 6% fast. If you decrease the pulse frequency by 12%, the speedometer will be right but the odometer will under record distance by 6%. You can have one right or the other - but not both. To get both correct you'd need to be able to reprogram the instruments on the bike, and it appears that there is no way to do that. It's fixed at the factory.


I re-measured the speedometer and odometer errors using a signal generator and FET switch to emulate the pulses from the speed sensor with a frequency counter (good to 0.1Hz) measuring the pulse frequency. The sensor sends 6 pulses per wheel revolution. What I found was that a pulse frequency of 122Hz (122 pulses per second) gave an indicated speed of 100mph. The speedometer was linear, with 61Hz showing a speed of 50mph and 30.5Hz showing a speed of 25mph. Setting the speed to exactly 60mph I measured the time taken for the odometer reading to increase by 3 miles. It took 63 seconds for the first mile, 126 seconds for the second mile and 189 seconds for the full three miles. Measuring the rolling circumference of the front tire (as described above) I estimated it at 193cm. Now we can calculate speedometer and odometer error.

At 122 pulses per second and 6 pulses per revolution, the front wheel will cover (122/6 * 1.93) = 39.24 meters in 1 second, which is 141.26 km in 1 hour. That's equivalent to 87.77 mph. So when the true speed reads as 88 mph, the speedometer will read 100 mph, which is an error of +11%.

At a constant 60mph on the speedometer, the odometer should increase by exactly 1 mile every 60 seconds. In fact it increased by 1 mile every 63 seconds. That's an error of 5%. The same conclusion applies with regard to correction. You can modify the pulse frequency to either make the speed correct or the odometer correct, but not both at the same time

So as you can see, the speedometer is 11% optimistic and the odometer is 5% optimistic when the front wheel has a rolling circumference of 1.93 meters. Those are pretty close to the numbers I got when actually riding (+12% and +6%).

Three wires connect the speed sensor with the instrument cluster, one green, one black with a red stripe and one brown with a white stripe. The green wire is ground, the black/red wire is +V (around 12v) and the brown/white wire carries the speed signal, a series of square waves (0 to 10v), 6 pulses per revolution of the wheel. I believe the way it works is that the sensor pulls the brown/white wire from the speedometer head low (ground) each time one of the magnets in the hub passes the hall effect sensor.

Various pulse frequency changing commercial units are available such as those from HealTech Electronics. While many are sold with wiring harnesses which plug straight into a bike, that won't be the case for the RX3, so you'll be cutting and splicing wires. Expect to pay at least $100 for a HealTech device. For $30 (plus shipping from Australia) you can get something from Jaycar. For around $80 you can get a SpeedoDRD

For those thinking of fitting a 19" front wheel, there's good news. A larger wheel will have a larger circumference (how much larger depends on the tire you fit of course). Going from a 110/80-18 to a 100-90-19 gives you about a 5% increase in circumference. Since the wheel has fewer revolutions per mile the speedometer will read 5% slower, but since it's already reading about 12% fast, 5% slower will make it more accurate! The same goes for the odometer. With the stock tires it records about 6% too many miles. With the larger tire you get more miles per revolution, so you'll get closer to the true mileage reading.

How to get Accurate speed and Distance

Well, you could add an aftermarket motorcycle computer with speed and distance readouts, such as one of those made by Trail Tech or Koso. That will cost you from around $120 to $300+ depending on which model you chose. Alternatively you could get a similar device designed for bicycles for anywhere between $5 and $50. The designed of the bicycle units is that they are almost always powered by an internal battery, so they don't have an option to leave the LCD backlight on at night. They also typically only read to 99.9 mph, which for the RX3 isn't an issue! The display size is typically smaller than the computers designed for motorcycles too. The bicycle models use a magnet and sensor to determine the wheel speed, not the bike's original sensor. They're designed for bicycle wheels and forks with relatively close spacing. However by using a stronger (rare-earth) magnet you can get them to work on the RX3 just fine.

Here's a couple of pictures. The sensor is glued to the inside of the fork and the rare-earth magnet is glued to the wheel rim:

I attached the speedometer head to the bars as shown in the last image:

You can find bicycle computers of all shapes, sizes and prices and rare earth magnets on ebay