The Yamaha R15 V3 is currently one of the best performance oriented bike on sale in India. Though it may have a low displacement 155 cc engine, it churns way more power than a regular 150 cc unit and the low kerb weight of the bike further helps with its performance. The bike can easily reach speeds around 140 km/h which is again a feat not many 150 cc bikes can achieve. However, the speed shown on the speedometer is not the true speed of the R15 due to the speedo error present on all vehicles. The video below by Suraj Verma shows a side by side comparison of the bike’s speed on speedometer and accoding to GPS.
As seen in the video, the speedometer of the R15 has a speedo error than ranges between 8-10 km/h from 0 km/h till its top speed. There are various reasons as to why the speedometers have errors. In fact, no speedometer can be 100 percent accurate. Most manufacturers deliberately build speedometers with a slight error that falls within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. This is done for safety reasons so as the rider rides a bit slower than what he actually sees on the speedo. It also helps in preventing a vehicle to be slapped with speeding tickets as the cameras on road capture the real speed based on GPS which is accurate.
As long as a car is maintained at factory specs, its speedometer continues to have a speedo error as implied by the manufacturer. However, if a vehicle is modified, its speedometer may need to be re-calibrated. This calibration must take into account several factors, including the ratios of the gears in the drive cable, the final drive ratio in the differential and the diameter of the tires. Tire modification are among the most common causes of increased speedo errors as bigger tires travel more and hence lead to a different reading on the speedometer. Re calibration is required to take care of this issue.
The speed test done in this video cannot be deemed as exactly accurate because the GPS speed shown also depends on the satellite signal at a given point of time. However, it does gives us a fair idea about speedo error on vehicles and about how much variance can be there. Professional equipment used during authorized tests have also shown a similar tolerance range of 1% to 5% on speedometers. GPS devices are positional speedometers and show result based on how far the receiver has moved since the last measurement. Also, its speed calculations are not subject to the same sources of error as the vehicle’s speedometer and signal quality is the only major factor affecting its result.