A calibrated wheel speed sensor gives the quicker, steadier speed reading on a bike, and it keeps working under trees, between tall buildings, on tight singletrack and indoors on a trainer. GPS needs no setup, works on any bike you ride, and on open roads at a steady pace it’s typically within a couple of km/h of your true speed. For most road riders and commuters, GPS is plenty. A wheel sensor earns its place if you ride under tree cover, care about sprint numbers, or train indoors.
This page is about live speed. If it’s ride distance that doesn’t match, see bike computer vs GPS distance.
How does each one measure speed? #
A wheel sensor counts wheel revolutions. The classic version is a magnet on a spoke and a pickup on the fork or chainstay; newer ones strap to the hub and use an accelerometer. Speed is revolutions times the wheel circumference you entered, divided by time. A road wheel is a little over 2 metres around, so at 30 km/h it turns about four times a second, and the reading updates with every turn. The sensor sends its data over Bluetooth or ANT+ to a bike computer, watch or phone app that supports it.
GPS works out your speed from the satellites. Most receivers calculate it from the Doppler shift of the satellite signals rather than from position changes alone, which keeps the number steady on an open road. Phones and bike computers usually update about once a second, and apps smooth several readings so the display doesn’t flicker.
Which is more accurate in each situation? #
| Situation | Wheel sensor | GPS |
|---|---|---|
| Open road, steady pace | Excellent, if the circumference is right | Very good |
| Sprint or hard acceleration | Responds within a wheel turn | Trails by a second or two; short peaks read low |
| Slow climb at 6-8 km/h | Fine | Noisier, because small errors are big relative to your speed |
| Tree cover, canyons, downtown | Unaffected | Weaker fix, jumpier reading |
| Tunnels and underpasses | Unaffected | Drops out |
| Indoor trainer | Works on a wheel-on trainer; smart trainers report their own speed | Useless |
| Several bikes | A sensor and a circumference setting per bike | One phone covers them all |
| Setup | Enter or measure the circumference | None |
Where a wheel sensor goes wrong #
- The circumference is wrong. Every percent of error in the setting is a percent of error in every speed, forever. The default for a tyre size can be a couple of percent off your real tyre, pressure and weight. A rollout fixes it; the steps are in our distance and calibration guide.
- The magnet has slipped. A magnet that has moved away from the pickup causes dropouts: the speed falls to zero, then jumps back.
- It’s on the wrong bike. Move the head unit to a bike with a different wheel and forget to switch profiles, and the numbers shift.
- The wheel isn’t gripping. On wet grass, mud or ice, the wheel can spin or skid, and the sensor reports the wheel, not the ground.
Where GPS goes wrong #
- Blocked sky. Leaves, rock walls and buildings weaken and reflect signals, so the fix wanders and the speed jumps.
- Low speed. Climbing at walking pace, a few metres of position error matters more. Doppler speed helps, but slow readings still wobble more than fast ones.
- Lag on sprints. Because apps smooth several one-second readings, the display trails a sprint by a second or two, and the peak of a three-second effort reads lower than it was. Our explainer on GPS speed lag covers why.
- Phone placement. A phone in a jersey pocket has your body between it and half the sky. On the handlebars it does much better.
Which should you use? #
| Rider | Best choice | Why |
|---|---|---|
| Road rider on open roads | GPS alone | Accurate enough, no setup |
| City commuter | Either; a sensor if you want steady readings between buildings | GPS jitters downtown, but speed averages out fine |
| Mountain biker under trees | Wheel sensor | GPS struggles under canopy and on tight turns |
| Indoor training | Wheel sensor or the trainer’s own data | GPS doesn’t work indoors |
| Sprint and pacing work | Wheel sensor | Instant response |
| Rental, bike share or several bikes | GPS | Nothing to fit or calibrate |
| E-bike rider checking the display | GPS | An e-bike’s speed display depends on its wheel-size setting, and GPS shows whether it’s right |
Plenty of riders use both: GPS for the route and a wheel sensor for the live speed. Some hub sensors paired with a GPS head unit can set their own circumference from GPS. If you’re weighing a phone against a dedicated head unit, phone vs bike computer covers screens, battery and group-ride numbers.
Using your phone as a bike speedometer #
If you don’t want a sensor at all, a phone on the handlebars does the job for speed. Speedometer GPS shows a big readout inside a dial that auto-ranges, so on a bike it sits on a fine 0-60 km/h (0-40 mph) scale, and an accuracy indicator shows the fix quality so you know when to trust the number. It records top speed from the smoothed value on purpose, so a single glitchy reading can’t give you a fake maximum, though a very short sprint peak is slightly understated.
Be clear about its limits. It uses only the phone’s GPS and doesn’t pair with Bluetooth or ANT+ sensors, so there’s no wheel-sensor speed or cadence. And its trip distance reads short below about 18 km/h (11 mph), because it ignores steps under 5 metres between one-second fixes to stop a parked phone adding distance. Live speed and average speed aren’t affected, but on slow climbs and easy rides don’t treat its distance as your ride total. Mount the phone firmly on the stem or bars, set everything up before you roll, and leave it alone while you ride. If you’re wondering what a cadence sensor adds, see cadence vs speed on a bike.
Frequently asked questions #
Is a speed sensor more accurate than GPS on a bike? #
A correctly calibrated wheel sensor is more consistent, responds faster and works where GPS struggles, under trees and indoors. On an open road at a steady speed the two usually agree closely. A sensor with the wrong circumference setting is wrong on every ride, which GPS never is.
Do I need a speed sensor if I have GPS? #
Not for most road riding or commuting. A sensor is worth adding if you ride trails under tree cover, train indoors on a wheel-on trainer, or want accurate sprint speeds. It’s inexpensive, and it needs a measured circumference to be accurate.
Why does my bike speed jump around on GPS? #
Usually a weak fix: tree cover, buildings, or the phone sitting in a pocket. Low speed makes it worse, because small position errors are large relative to your speed. Mount the phone on the handlebars with a clear view of the sky.
Why does GPS show a lower top speed than my speed sensor? #
GPS updates about once a second and apps smooth several readings, so a short peak gets averaged down. A wheel sensor updates every turn of the wheel and catches the peak. On long, steady descents the two top speeds are much closer.
Can a phone’s accelerometer replace a speed sensor? #
No. An accelerometer measures changes in speed, and adding those up drifts badly within seconds. Phone speed on a bike comes from GPS; for speed that doesn’t need the sky, you need a wheel sensor.