Most riders know their dashboard speedo runs a little optimistic, the same way a car's does. What's less well known is that motorcycle speedometers, on average, drift further from true ground speed than car speedometers do — and the reasons are specific to how bikes are built and modified.
The same legal over-read rule applies to bikes
In the EU, UK, and many other markets, vehicle speedometers — bikes included — are required by regulation to never indicate a speed lower than the vehicle's true speed, but are permitted to indicate higher. Manufacturers build in a margin to comfortably stay on the right side of that rule across the tolerance range of mass-produced components, which is why a brand-new, stock motorcycle speedometer will typically over-read by a small, predictable amount even when everything is factory-correct.
Why bikes drift further than cars
A car's four wheels and relatively large tyre diameter make small dimensional changes proportionally less significant. A motorcycle's smaller wheel diameter means the same absolute amount of tyre wear represents a larger percentage change in rolling circumference, which feeds directly into a larger speedometer error if the bike calculates speed from wheel rotation.
Gearing changes compound this further. Many riders swap the front or rear sprocket to alter acceleration or top speed — a common, simple modification. But on bikes where the speedometer signal is taken from the gearbox output shaft or the final-drive sprocket rather than an independent wheel sensor, changing the sprocket ratio changes the relationship between that signal and actual road speed, throwing the speedometer reading off by whatever percentage the gearing changed, on top of any factory over-read that was already built in.
How much it actually skews
A stock motorcycle with standard tyres and gearing typically over-reads by roughly 3 to 7%, similar to a car. With a worn rear tyre (smaller effective rolling radius) or a sprocket swap toward shorter gearing, that error can grow into double digits — riders who've changed gearing for track use sometimes find their dash reading 10% or more above true speed, which on a bike already running near a track's posted limits is a meaningful gap.
Using GPS as an independent check
Because a GPS-based motorcycle speed checker measures your actual change in position rather than reading any signal off the drivetrain, it's completely unaffected by tyre wear or gearing changes — it reports the same true ground speed regardless of what's been modified on the bike. Riding the same stretch of road with both your dash and a GPS speed checker running side by side, on a private road or closed circuit, is the simplest way to find your bike's actual error margin and adjust your mental "subtract X" number accordingly.
The takeaway
Motorcycle speedometers inherit the same legal over-read margin cars have, but tyre wear and common gearing modifications push that margin further on two wheels than on four. A GPS check is unaffected by either factor, which makes the motorcycle speed checker a useful independent reference, especially after any tyre or sprocket change.