The Moveee team
Free coaching · a real route for every ride
A wheel speed sensor does not measure speed. It measures how many times your back wheel went round, and how long that took. Turning that into a speed needs one more piece of information, and it is one only you can supply: how far the bike travels in one revolution of that wheel.
Get it right and your speed is accurate to within a percent or so. Get it wrong and every number from the session is wrong by the same proportion — speed, distance, and how far up the route you appear to be — while looking entirely plausible. Nothing on screen will tell you. This is the single highest-value setting in a speed-sensor indoor setup, and it takes two minutes.
How wrong can it be?
More wrong than people expect. If you leave the road default of 700×25c in place but your turbo bike is an old mountain bike on 29×2.2 tyres, you are out by about 10.5%. An hour that you rode at a genuine 27 km/h reports as 24.4. Over that hour you "travelled" three kilometres less than you did.
Ten percent sounds tolerable until you notice what it does to comparison. Ride the same route in January and again in March on the same wrong setting and the comparison is fine — both are wrong by the same amount. But compare an indoor ride to an outdoor one, or to a friend, or to the route's stated distance, and nothing lines up, and you will waste an evening wondering whether your sensor is broken.
The common sizes
Rolling circumference by tyre
| Tyre | Circumference | Where you see it |
|---|---|---|
| 700 × 23c | 2096 mm | Older road setups, and still common on turbo-only bikes. |
| 700 × 25c | 2105 mm | The modern road default, and what Moveee assumes if you say nothing. |
| 700 × 28c | 2136 mm | Endurance road and light gravel. A 31 mm difference from 25c — about 1.5%. |
| 700 × 32c | 2155 mm | Gravel and commuting tyres. |
| 650b × 47 | 2085 mm | Smaller rim, fatter tyre — lands near a 700×23c, which surprises people. |
| 26 × 2.0 | 2055 mm | Older mountain bikes, often what ends up on the turbo. |
| 29 × 2.2 | 2326 mm | Modern MTB. Over 10% out from a road default — the worst mismatch to leave unfixed. |
Nominal figures for a tyre at riding pressure. Your own wheel can sit a percent either side — roll it out below if you want the exact number.
These are good defaults, and for most people picking the right row is the end of the job. Moveee tells you which one it is using the moment a speed sensor pairs — a line that says, for example, "using a 700×25c tyre (2105 mm)" — precisely so that a mismatch is obvious in the first two seconds rather than after a season.
Measuring it properly, if you want to be exact
The table assumes a nominal tyre at a nominal pressure. Your actual rolling circumference depends on the real width of that tyre on your rim, your pressure, and your weight squashing it. If you want the number rather than an estimate, do a roll-out — it takes longer to read this paragraph than to do it.
Inflate to what you actually ride
The pressure you will use on the turbo, not the one printed on the sidewall.
Sit on the bike
This matters. An unweighted tyre rolls out longer than a loaded one — a percent or more on a fat tyre.
Mark the floor at the valve
Put the valve at the bottom and mark the floor beneath it with chalk or a strip of tape.
Roll forward exactly one revolution
In a straight line, until the valve is back at the bottom. Mark the floor again.
Measure between the marks, in millimetres
That is your number. Do it twice and average the two if you want to be fussy.
Put that figure into Devices → wheel size as a custom value. You will not need to touch it again unless you change tyres.
Two things that quietly change the number
A trainer tyre. Plenty of people fit a dedicated turbo tyre for the winter — usually a hard, slick compound in a nominal size that may not match what was on there before. Change the tyre, re-check the setting.
Pressure drift. A tyre left on the turbo for a fortnight loses pressure, gets squashier, and rolls out slightly shorter. This is a fraction of a percent, not a problem, but it is the reason a roll-out done at your real pressure beats one done on a freshly pumped tyre you then never top up again.
Where this does not matter
If you ride a smart trainer or a power meter, none of this applies to your power figures — watts are measured directly, not derived from wheel speed. Tyre size still affects the speed a wheel-on smart trainer reports, but the training numbers you actually care about are unaffected.
For a speed-only setup, though, this one field is the difference between a session you can trust and a season of numbers that are quietly 10% wrong. Set it once, properly, and then forget about it.
Sources 4
Where this article summarises a study, the study itself is linked — not a write-up of it.
- 1 Wheel Size and Circumference (Edge 830 Owner's Manual) · Garmin · 2019
- 2 Dell'Orto G, Ballo FM, Mastinu G, Gobbi M, Magnani G Racing bicycle tyres – Influence on mechanical characteristics of internal pressure, vertical force, speed and temperature · European Journal of Mechanics - A/Solids · 2023
- 3 Grappe F, Candau R, Barbier B, Hoffman MD, Belli A, Rouillon JD Influence of tyre pressure and vertical load on coefficient of rolling resistance and simulated cycling performance · Ergonomics · 1999
- 4 Cycling Speed and Cadence Service 1.0 · Bluetooth SIG · 2012
Your own roads, indoors, in a browser tab
Moveee Indoor pairs your smart trainer over Bluetooth and rebuilds a route you have saved as a 3D road — real gradients, ERG for the intervals, and the ride saved with full power data. Nothing to install. Free while it is in alpha.
Start an indoor rideKeep reading
Indoor cycling app with no power meter: what you get and what you don't
Both columns side by side. What a speed sensor gives you indoors — the route, distance, heart rate, cadence, a ride that counts — and what it cannot: watts, ERG, TSS, FTP. Including why we refuse to show you a virtual power number.
How to use an indoor cycling app without a smart trainer — a speed sensor is enough
A £30 wheel speed sensor turns any dumb trainer into a real route in a 3D world, with the ride saved to your calendar. What you need, how to set it up, and the one thing about hills you should know before your first session.
Gears explained: what the numbers on your cassette mean
Tooth counts to gear ratios to speed at a cadence, why steps matter more than the number of gears, what cross-chaining costs, and how to pick a gear on a climb.
Tyre pressure for cyclists: the numbers that actually make you faster
Harder is faster right up until it isn't — and the point where it stops is lower than the number printed on your sidewall. Rolling resistance, impedance and how to find your own breakpoint.
Cycling in cold weather: how to dress for 0 °C to 15 °C
Riding makes its own wind, which is why the forecast temperature isn't the one you're dressing for. A layering table by temperature, why extremities are the real problem, and the signs you've got it badly wrong.
Speed sensor or smart trainer: when is the upgrade actually worth it?
Four signs you have outgrown a speed-only setup, what power actually unlocks, and three routes compared — including the one most people overlook, which is a power meter rather than a trainer. Plus the honest case for staying where you are.
Gearing for climbing: the arithmetic nobody does
The belief that low gears are an admission of weakness has cost riders a great many good days. A table of what your lowest gear actually forces your legs to do at climbing speed — and why a 34×28 on a steep road is a leg press with a view.
Where the bike lives decides how often you ride it
A bike that takes four minutes and two decisions to extract gets ridden less than one you can lift off a hook. Storage options honestly compared, why damp beats cold as the enemy, and the trainer corner that actually gets used.
Disc brake noise: what it means and what to do
Better than rim brakes in nearly every way that matters and worse in exactly one — when they are unhappy they tell you loudly, in front of other people. Diagnosis by sound, the five-minute alignment fix, and the one noise that means stop riding.
Tyre width and rolling resistance: why 28 mm beat 23 mm
The narrow-is-faster rule was true on a steel drum and wrong on a road. What actually changes when a tyre gets wider, where impedance losses take over from rolling resistance, and the width at which aerodynamics finally starts charging you.
Cycling with type 1 diabetes: fuelling, monitoring and what riders do
Steady aerobic riding pushes glucose down and a hard interval can push it up, which makes a varied ride genuinely difficult to manage. What the international consensus guidance describes — and why every number here belongs to your own care team, not an article.
The perfect 60 km: designing a mid-week loop you'll ride all year
The route you ride fifty times a year is a training tool. One climb in the right place, a 15-minute uninterrupted stretch, few junctions, the same direction, and built for winter.