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26-06-2024 | Frank Jansen

Why "40 degrees on the Garmin" is often inaccurate

You often read on Strava: "It was 40 degrees on my Garmin." No one will deny that it was indeed hot. However, those 40 degrees often don't tell the whole story. What's the truth behind them?

temperature bike computer
Photo: During my ride today, my bike computer indicated nearly 33 degrees. The actual measured temperature was 28 degrees.

Temperature is measured in the shade

It's simple: temperature is measured in the shade. This ensures an accurate representation of the air temperature without direct sunlight influencing it. When your bike computer is exposed in an open field, sunlight heats the housing significantly, resulting in a (much) higher reading. How much higher? It varies based on factors like sun strength, cloud cover, bike computer brand, and sensor location. From my own observations, the difference can range from as little as 4-5 degrees to even more.

"But in the sun it was 40 degrees."

No, you can't measure temperature in the sun. It's like trying to measure wind speed indoors. While it may feel much warmer in the sun due to its direct influence, the actual temperature is best measured in the shade. The perceived warmth varies based on factors like the sun's position (and therefore the season), wind, humidity, and more. One thing is clear: relying on your bike computer for an accurate temperature reading in the sun is not reliable.

What about wind chill?

Everyone is familiar with the concept of wind chill from winter. Interestingly, you can also calculate wind chill in summer. It depends on two factors: humidity and temperature (measured in the shade, of course). The number on your Garmin doesn't provide an accurate wind chill reading either.


Once you start cycling in the sun with your bike computer, the temperature reading becomes immediately unreliable (or technically invalid, to be precise). So, it's best to disregard it!

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