What is a motorcycle GPS speedometer?
Bike speedometers are the most optimistic instruments on the road. A 10% over-read is normal and completely legal. This page shows what you're actually doing, on a dial that reads to 300, plus a standing-start timer and the lean angle your line implies.
Why do bike speedos read so high?
Same rule as cars: never low, up to 10% plus 4 km/h high. But manufacturers lean on that allowance harder on bikes, and there are mechanical reasons for it.
A bike usually takes its reading from the front wheel or the gearbox, and both are more sensitive than a car's. Change your sprockets and you change the gearbox ratio directly. Fit a taller rear tyre and the rolling radius shifts by a bigger percentage than it would on a car. And a bike tyre is round in section, so its effective radius shrinks as you lean. The more angle you carry, the more your speedo drifts.
How is the lean angle worked out?
It's calculated, not measured, and that difference matters.
A phone strapped to a leaning bike does not read the lean. It leans with the bike, and its accelerometer sees gravity and cornering force added together, which points straight down through the machine in any steady corner. Same reason you don't feel like you're falling over mid-corner. A tilt sensor on a bike reads close to zero however hard you're leaning.
So we work it out from the physics instead. In a steady turn, lean follows from your speed and how fast you're turning: tan θ = v·ω / g. Both come from GPS, so there's no extra permission and no sensor to fool.
Treat it as a good estimate rather than a measurement. GPS heading is noisy at low speed, the number is heavily smoothed, and it assumes a steady balanced turn, so it won't follow a quick flick through a chicane. It's genuinely useful for seeing how much angle your favourite corner really takes. It's not a scoreboard.
Using it on the bike
- Mount it properly. Bar vibration is hard on phone cameras. A mount with rubber damping is worth having on anything with a big single or a twin.
- Bikes get great reception. No roof, no metallised screen. Accuracy usually settles faster than it does in a car.
- Set it all before you ride. Unit, alert, everything. There is no good version of poking at a phone on a moving motorcycle.
- Glare. The instrument stays dark in both themes, which reads better in direct sun than a white one.
Checking a speedo corrector
Changed sprockets or fitted a corrector? This is a quick way to verify it. Ride a steady indicated 100 on a straight road with accuracy under ±10 m, hold it fifteen seconds, and read both. Then do it at 60 and 130. A properly set corrector gives you the same small percentage error at every speed. An offset that shows up at one speed but not another means something else is going on.
Frequently asked questions
How much does a motorcycle speedometer over-read?
Usually 7 to 10% on a modern bike, sometimes more. It's legal and deliberate. The rules allow up to 10% plus 4 km/h over and forbid reading under, and manufacturers use most of it.
Does my phone measure the lean angle?
No, and it couldn't. A phone on a leaning bike leans with it, and its accelerometer only sees the combined force, which points down through the machine. We calculate lean from speed and rate of turn instead, which is how the angle actually comes about.
Why is the lean zero on a straight road?
Because you're not leaning. The angle comes from how fast your heading is changing, so no turn means no lean. It also stays quiet below walking pace, where GPS heading is too noisy to report.
Can I use this after a sprocket change?
That's one of the best uses for it. Changing final drive shifts the speedo directly, and comparing indicated against GPS at a few steady speeds tells you exactly how far out you now are.
Does vibration affect the reading?
Not the speed, which comes from satellites rather than a sensor. It's hard on the phone though, so use a damped mount.