GpxFix Blog

Fixing a Bike Ride: Tunnels, Fast Descents and Impossible Speed Spikes

By Roy Lachica on October 22, 2026. Last updated October 22, 2026.

Riding breaks GPS recordings in ways running does not. You cover four times the ground in the same hour, you go through tunnels and under bridges, you descend at 60 km/h through trees, and your bike computer may be getting distance from a wheel sensor rather than from satellites. The result is a familiar set of complaints: a max speed of 124 km/h, a straight line where a tunnel was, 300 metres of climbing that never happened, and a distance that disagrees with your bike computer. Here is what causes each one and what to do about it.

Tunnels, underpasses and bridges

In a tunnel the receiver loses the sky. What happens next depends on the device: some stop writing points until they regain a fix, leaving a gap and a straight line across it, others keep writing the last known position, producing a cluster of identical points followed by a jump when the fix returns. Both look bad, and the jump is what generates an absurd top speed, the mechanism is described in GPS freezes then jumps.

Tunnels also upset barometric elevation. Air pressure inside a tunnel or under a large structure is not the same as outside, so a barometric altimeter can record a climb or a drop that never happened, and those metres land in your total.

Fast descents through trees or terrain

At descending speed, a position error that would be trivial at running pace becomes a visible kink in the track. Add tree cover or a valley wall blocking half the sky and you get a track that cuts corners, oscillates across the road, or picks up short backwards hops. On a long descent this is also where the worst speed spikes come from, because a sideways error of 30 metres between two points one second apart reads as a huge instantaneous speed.

Urban canyons and the group ride

City riding bounces signals off buildings, which puts your track on the wrong side of the road, in the tram tracks, or inside a building. This is the classic cause of a ride that looks fine at a glance but fails to match the segments you know you rode, see why your Strava segment did not register. Industrial areas, bridges with steel structures and, less obviously, wind turbines can all add local interference.

Wheel sensor distance versus GPS distance

If your head unit is paired with a speed sensor, the distance it displays comes from wheel revolutions, not from GPS. That number is usually the more accurate one, and it is also the one that will not match what a GPX file measures from coordinates, especially if the wheel circumference calibration is off. This is a measurement difference rather than an error, and it is worth understanding before you try to fix anything, see why Garmin and Strava sometimes show different distances.

Recording interval

Smart recording saves battery by writing a point only when the device decides something changed. At cycling speed, that can mean 40 or 50 metres between points, which rounds off corners, shortens distance and starves segment matching of geometry. For rides where the data matters, use one second recording and accept the battery cost.

What to do with a ride that came out wrong

1. Look before you fix. Upload the file to GpxFix and check the summary and the speed chart. A single tall spike is a different problem from a track that is jittery from end to end.
2. Clear the obvious damage. Use the 'Fix GPS issues', or the 'Just fix it'-button, which removes spikes and unrealistic speeds.
3. Put the track back on the road. For road riding, Snap to road is the strongest tool available, it aligns points to real road geometry while keeping your timestamps. Choosing between that and the other smoothing modes is covered in which smoothing method to use.
4. Deal with the tunnel. If points are missing, restore the section rather than leaving a straight line, see restoring missing GPS data. If you have a clean file of the same route, use it as a reference file.
5. Fix the elevation. Tunnel and weather-driven pressure errors are what elevation calibration is for, and why your elevation gain is wrong explains how the total gets inflated in the first place.
6. Trim the cafe stop or the drive home if the ride kept recording, see forgot to stop your watch.

Recording a cleaner ride next time

• Mount the computer where it can see the sky, on the stem or bars rather than under a bag or inside a jersey pocket.
• Use one second recording for rides you care about measuring.
• Enable dual-band or multi-band GNSS if your device has it, the accuracy difference in cities and valleys is real, see the GPS and elevation accuracy comparison.
• Calibrate your speed sensor wheel size if you rely on sensor distance.
• Let the device get a fix before you roll, the first minute of a ride is where a surprising number of errors originate.

Frequently asked questions

Why does Strava show a max speed I never hit?

Because max speed comes from the shortest interval in the file, so one bad pair of points sets it. It is the single most common cycling data complaint and it is almost always a GPS artefact rather than a recording of your actual descent.

Will smoothing my ride shorten the distance?

Usually slightly, yes. Removing jitter removes the extra wiggle length that the jitter added. If the distance matters to you, read why your activity distance is wrong before deciding how much smoothing you want.

Can I fix a ride recorded entirely without GPS, on a trainer?

An indoor ride has no coordinates to repair, but it can still be loaded, inspected and corrected in other ways, such as its timestamps or its heart rate data.

My ride is in completely the wrong place on the map

That is drift on a different scale, see manually fixing a track that drifted far off course.

Got a ride with a tunnel jump or a nonsense top speed? Upload it to GpxFix and see what the chart says. Related reading: understanding GPS spike zones and why cadence values come out halved or doubled.