Garmin speed sensor distance and speed accuracy

I bought a Garmin speed sensor several years ago as a recommendation from my triathlon buddies. When the battery died out for the first time, I rode a couple of times without the sensor and wondered how the speed was affected by this on my Garmin GPS unit. I did some research and I can now share with you all what I learned about Garmin speed sensors.

Guess what! it’s not just about speed data… and accuracy is part of the topic.

Does Garmin speed sensor measure distance?

Garmin speed sensor measures and records the distance. Indeed, the sensor is placed on the hub of the wheel. Coupled with the wheel diameter, the sensor can tell the distance traveled by the bike each time the wheel fully rotates. It is more accurate than using GPS data.

The wheel diameter is an input that can be specified within your GPS bike computer or that can be automatically calculated. Thanks to this diameter, the speed sensors will be able to determine the distance you travel, and therefore your speed, even if the GPS signal is bad. The measurement only relies on the wheel diameter and the rotation of the sensor attached to the wheel.

Does Garmin sensor override GPS data?

When using a Garmin sensor, the speed and distance recorded will come from the sensor, not from GPS data. So on your bike computer or watch, what you see when riding is data from your sensor and not the GPS data. Both speed and distance data are overwritten.

Is Garmin speed sensor more accurate than GPS?

Garmin speed sensor is more accurate for speed and distance than GPS. It doesn’t rely on GPS signal, only on the wheel rotation. It works at all times, even in remote locations, and especially on steep climbs where your speed can be too slow for the GPS to be sure you are riding.

Generally, you won’t notice the difference in accuracy between the GPS signal and a speed sensor on the ride summary. If the GPS signal is good where your ride, the overall accuracy is similar to using a speed sensor. The real difference is on hills, remote areas, or on specific segments. GPS signal can simply be scarce if you ride on a road covered by trees. For hills, the steepest they are, the slower you will ride, until you reach the limit of the GPS data accuracy.

Whereas it is nothing to worry about on the macro scale (i.e. the total distance of the ride), Garmin speed sensor will give a better reading on the micro-scale. It can be great to track your speed on a very specific short interval, like a short but steep hill.

Having this kind of detail on your ride can be precious if you are really looking at improving your performance.

How does Garmin speed sensor know wheel size?

Garmin speed sensor determines the wheel size based on manual input or automatic calculation in your Garmin GPS device (bike computer or watch). In this case, the device will reverse engineer the wheel size by matching the sensor speed to the GPS speed.

Indeed, Garmin speed sensor

Sensor’s speed = Angular velocity (rpm) x Wheel radius

As the speed from the GPS and speed sensors must match, then:

Wheel radius = GPS speed ÷ Angular velocity (rpm)

This calibration process will usually occur when the GPS signal is very good. Once that’s done, the wheel diameter is known for using the speed sensor’s data instead. It will then be more accurate when the GPS signal is bad.

Note: rpm means rotation per minute

Where to put Garmin speed sensor?

You can put your Garmin speed sensor on either the front or the rear wheel hub. Your metrics will be identical since both wheels rotate at the same speed. Many people put it on the rear because they consider it more accurate as the rear wheel generally remains in contact with the ground more often.

But, if you are a regular road cyclist, putting your speed sensor on the front wheel won’t make a difference. A slight difference may only occur if you lift your front wheel from time to time which may happen to mountain bikers that jump to go over wooden branches and rocks.

When jumping, we tend to lift the front wheel first and put it back on the ground last. So putting a speed sensor on the rear wheel will be more accurate if you do a lot of jumps.

How long does Garmin speed sensor battery last?

Garmin speed sensor’s battery last on average 1 year, it may vary from 6 months to 2 years depending on how often your ride. Garmin considers that it will last a year if you ride 1h per day. So that’s 7h per week of riding. Additional factors can affect the battery life such as temperature.

As with any battery, the temperature can affect its life. In the case of the Garmin speed sensor, it is supposed to operate from -20º to 60ºC (from -4º to 140ºF), but high temperatures tend to reduce battery life.