Really dumb question: gps vs speed sensor

So I have just been using my garmin gps for speed readings, i never have any reason to believe it isn’t close enough, especially when I’m just riding to power. But I’m going for a personal best in a few weeks and was thinking of using the speed sensor for the sake of accuracy. Anyhow my dumb question is whether gps speed readings are more likely to over or under estimate true speed versus a sensor or if it’s a wash over a long enough course


A speed sensor will get you better resolution if you’re trying to measure top speed, but it won’t matter over any sort of distance if you’re trying to measure average speed.

In fact, errors in wheel circumference, affected by the measurement itself, in addition to fluctuating tire pressure and road conditions, will actually compound and give you an inaccurate average speed and distance traveled.

GPS is the way to go.


Speed sensors are great if you are going to be in areas with poor GPS connectivity. RIding in cities, canyons, forest roads, etc. As long as you have a good lock, GPS will probably be better.

A friend of mine recently put a speed sensor on his MTB to try and be more accurate. On one of our races we both had nearly identical distance readings. I was surprised since I figure the GPS would lose a little in tight corners and under the trees, but even if it did, not enough to warrant using a speed sensor in the future.

I have one for my dumb trainer just to track wear and tear.

That’s why I have them, any minor differences between gps or sensor doesn’t alter the stats enough to matter. Whereas not getting any data cos I’m playing in the woods does matter.

That and they are often bundled with other sensors so might as well use them!


This. In the woods, especially on twisty singletrack, while there’s full leaves on the trees, can give comically inaccurate GPS data. A speed sensor helps a lot with getting accurate speed and distance, even if the bread crumb trail is wrong.

If you’re mainly riding on roads with good visibility of the sky, you will get most errors evening out.

Also, not all GPS is equal. First there’s different services (plain old “GPS” run by US military, GLONASS, Galileo, etc). Second, the computer you use has some effect on accuracy. So do things like weather. So there’s several factors at play.

Chances are It’s going to be more than fine for your use. Even if you get a bad data point, there’s ways to fix that with GPX or FIT file editors

gps on road bike
speed sensor on mtn bike.

I’m not convinced it’s gps quality due to tree canopy. I think it’s just the frequency of the pings vs. the direction changes on a mountain bike. The computer tends to draw straight lines between points. On the road or a relatively “straight” course, those dots are pretty accurate. When you are going up switchbacks on a mountain bike, that line may miss the part where you actually turned around.

Both are different and you need to understand pros and cons… and what you are measuring.

Speed sensor will multiply the number of revolutions with the wheel circumference.
For the sake of simplicity if a wheel circumference is 2 meters it will turn 10000 in a 20km ride. If your wheel circumference is wrong by 1mm you have an error of 10 meters per 20km. You can realistically achieve better than 50m per 20km which in my opinion is pretty good. The trick is to have a good circumference measurement.

Keep in mind that the speed sensor measure a 3D distance - the distance travelled by the wheel.

The GNSS (aka GPS) is a different type of measurement. The first consideration is to know what distance is measuring (you will find this difficult to nail it in most cases) is it a 2D, 3D? How often is the sampling? What’s the receiver precision?
GPS is also prone to bad reception and notoriously bad estimating your vertical position.

When correctly used the speed sensor is simpler and more precise IMHO. GPS is equivalent in the open sky (no valleys, trees, tunnels…) and in a fairly straight route (the boring ones).

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Use a speed sensor. GPS is blunt instrument, Why Every GPS Overestimates Distance Traveled

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One more thing. Regardless of sensor used, recording sampling rate matters too. You want your computer to be recording every second, not every 5 seconds or “smart recording”. Storage is not an issue nowadays so there’s no point in having a slower rate

Speed sensor uses 3D distance. GPS doesn’t.

The only exception is when you are in areas with spotty GPS reception like tunnels or when you on a mountain side that is covered in trees. If you have a speed sensor, I’d put it on. But if a speed sensor weren’t part of my Wahoo Elemnt Bolt sensor bundle, I probably wouldn’t have bought one.