Most cycling speed problems come down to one thing: a wheel-magnet computer that was never set to the right wheel size. This tool sidesteps that entirely. It reads your position from your phone's GPS receiver several times a second and works out ground speed from how far you actually move, so there is no magnet to align, no spoke sensor to pair, and no tire circumference to type in. Open it, prop the phone on your handlebars, and your speed appears in km/h or mph.
No magnet, no spoke sensor, no wheel-size guessing
A traditional cyclocomputer counts wheel rotations with a magnet on a spoke and a sensor on the fork, then multiplies each rotation by the wheel circumference you entered during setup. Get that number wrong and every reading is wrong: a 700x25c road tire and a 700x32c gravel tire differ by enough that the same ride can read several percent apart. Many riders never measure their rolling circumference and just pick the closest preset, which bakes in an error for the life of the computer.
GPS does not care about your wheel at all. It measures the distance your bike moves across the ground over time, so the reading is the same whether you swap tires, run lower pressure, or move the same phone from your commuter to your mountain bike. There is nothing to calibrate and nothing to re-enter when your setup changes.
Works on any bike, including rentals and bike-share
Because nothing is bolted to the frame, the same phone reads speed on whatever you happen to be riding. That is genuinely useful for bike-share and rental bikes, where you cannot install a sensor and have no idea what wheel size the operator configured. It is also handy for anyone with more than one bike who does not want to buy and pair a separate head unit for each.
Touring riders get the same benefit on the road. If you fly with a folding bike or rent something at your destination, you still have a working speedometer the moment you can mount your phone, with no parts to carry or fit.
Checking an e-bike's assist cutoff
E-bike motors are legally required to stop adding power above a set speed: 25 km/h (about 15.5 mph) for EU pedelecs, 20 mph for US Class 1 and Class 2, and 28 mph for Class 3. The bike's own display drives that cutoff from its wheel-speed sensor, so if that sensor is miscalibrated or has been tampered with to read low, the motor can keep assisting past the legal limit without the rider knowing.
An independent GPS reading lets you sanity-check what the e-bike's screen claims. Ride at a steady pace on flat ground and compare the two. If the e-bike shows 25 km/h but GPS says you are doing 30, the bike's speedometer is reading low, which is exactly the symptom of a tuning device or a wrong wheel-size setting. This is a quick honesty check, not a substitute for legal compliance.
Where GPS is strong and where a wheel sensor still wins
GPS is accurate and steady at normal riding speeds out in the open, which covers the vast majority of road, gravel, and trail riding. It is less precise at very low speeds and during stop-start moves like trackstands, tight switchbacks, or crawling through traffic, because small position jitter matters more when you are barely moving. A wheel sensor handles those low-speed moments better since it counts real rotations.
GPS also needs you to actually move across the ground, so it cannot read your speed on an indoor trainer or rollers where the bike stays put. For turbo sessions, a trainer's own speed or power readout is the right tool. For everything outdoors, GPS is the simpler, calibration-free choice.
km/h or mph, and reading the screen on the bars
Cyclists in most of the world think in km/h, while riders in the US and UK usually want mph, so the tool toggles between them with one tap and keeps your choice. Nothing about the underlying measurement changes; only the units on screen do.
For a usable handlebar readout, mount the phone where you can glance at it without taking a hand off the bars, and turn screen brightness up so it stays legible in direct sun. Continuous GPS and a lit screen draw real battery, so on longer rides keep the phone topped up or carry a small power bank, and consider a stem or bar mount rather than balancing it loose.
Frequently asked questions
Do I need a wheel sensor or magnet?
No. This reads speed from your phone's GPS, which measures how far you move across the ground. There is no spoke magnet, no fork sensor, and nothing to pair or mount to the frame.
Do I have to enter my wheel size or tire circumference?
No. Wheel-size calibration only matters for magnet-based computers that count rotations. GPS ignores your wheel entirely, so swapping tires or moving the phone to another bike changes nothing about the reading.
Is it accurate enough for cycling?
At normal riding speeds in the open it is accurate and steady, which is fine for almost all road, gravel, and trail riding. It is less precise at very low speeds and during stop-start maneuvers, where a wheel sensor does better.
Can I use it to check my e-bike's speed limit?
Yes. Ride at a steady pace on the flat and compare GPS to the e-bike's display. If the bike reads noticeably lower than GPS, its speed sensor is likely miscalibrated or tampered with, which can let the motor assist past the legal cutoff.
Does it work on an indoor trainer or rollers?
No. GPS needs the bike to actually move across the ground, and on a trainer it stays in one place. Use the trainer's own speed or power readout for indoor sessions; GPS is for outdoor riding.
Can I switch between km/h and mph?
Yes. One tap toggles units and your choice is remembered. Use km/h if that is how you ride or mph for the US and UK; the measured speed is identical either way.