Glance down mid-ride and it's common to see two different numbers: your bike computer says one speed, your phone propped in a mount says another. Neither display is broken. They're measuring speed in fundamentally different ways, and each has its own failure mode that the other doesn't share.
How a wheel-based bike computer actually measures speed
A traditional wired or wireless bike computer doesn't measure speed directly at all. A magnet on your spoke passes a sensor on your fork once per wheel rotation, and the computer counts rotations per second. To turn that into a speed, it multiplies rotations by a wheel circumference value you entered (or that came pre-set from the factory) when you set the unit up.
That circumference number is the entire system's weak point. It's usually a generic figure for your tyre size, not a measurement of your actual wheel. Tyre pressure, tread wear, and even rider weight change the real rolling circumference by a percent or two, and if the wrong number was entered in the first place, the error is consistent and silent — the computer will confidently report a speed that's wrong by the same margin on every single ride.
How GPS measures your cycling speed
A phone's GPS-based cycling speed checker works completely differently: it has no idea your wheel exists. It tracks your actual position in space, several times a second, and calculates how fast that position is changing — either from the Doppler shift of the satellite signal or by measuring the distance between consecutive GPS fixes. It doesn't care about tyre size, tread wear, or rider weight, because it never has to know any of those things.
Which one drifts more over a ride
This is where the comparison gets interesting. A wheel sensor with a slightly wrong circumference produces a systematic error: it's wrong by roughly the same percentage all the time, on every ride, forever, until you correct the setting. GPS, by contrast, produces random noise: brief dips or spikes caused by tree cover, tunnels, or tall buildings, but no built-in bias. Over a long, open ride, GPS speed tends to average out close to your true speed; a wheel sensor with a 2% circumference error will be 2% off on every single ride, in the same direction, without exception.
The trade-off is that GPS noise is more visible moment to moment — a brief signal wobble under a bridge can make the number jump for a second — while a miscalibrated wheel sensor looks perfectly smooth and confident even when it's wrong.
When to trust which
If you've carefully measured your wheel circumference (roll the tyre exactly one rotation and measure the distance, rather than trusting a generic chart value) a wheel sensor is excellent for instant, dropout-free responsiveness, especially on technical trails with tree cover where GPS can wobble. If you haven't calibrated it, or you've swapped tyres recently, GPS is the more honest number, because it has no setup step to get wrong in the first place. Many riders use both: the wheel sensor for the live number on the bars, and an occasional GPS speed check to confirm the wheel sensor hasn't drifted.
The takeaway
Neither method is universally "more accurate" — they fail differently. A wheel computer fails quietly and consistently if its setup is wrong; GPS fails visibly and randomly when the sky view is blocked. If your two numbers disagree by a fixed amount on every ride, suspect your wheel circumference setting first, and use the GPS-based cycling speed checker as the independent reference to check it against.