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Daily Archives: December 2, 2021

I Finally Did the Math on How Much I’m Getting Ripped Off By My Garmin Speed Sensor on Strava… It’s A Lot

Don’t tell anyone, but I’m a bit of a geek when it comes to cycling. I do most of my own maintenance and repairs on a veritable fleet of bikes. I studied my shifting patterns to match the right cassette to my chainrings to my style of cycling on each bike. I picked tires based on wear patterns, durability and speed bleed. I picked clothing based on performance first, but the colors of my bikes played a prominent role in my final choices… and then I went and documented all of that here (along with a whole bunch of content about recovery) in over two million words over a decade. The point is, I do my homework.

And so I did with my trainer and speed sensor because I knew I was faster than I was getting credit for… I knew I was getting robbed. And like most, I don’t like getting robbed. I had to know by how much – while knowing just by feel, it was a lot.

So, I set up my stop watch and picked a fifteen second section, counting the revolutions of my crank to determine my cadence (88 rpm), while keeping an eye on the time, whilst (and at the same time) keeping tabs on my speed as shown on my Garmin head unit (18-mph give or take). Then I went to Sheldon Brown’s gear ratio calculator and entered my equipment (50/34 chainrings, 11/28 Shimano10 sp cassette, 172.5 mm crank arms…) and selected speed at 90 rpm… then went through and checked my speed at 90 rpm in the proper gear. 23.5-mph at 90 rpm, rounding down to 23 for those extra two rpm.

With a little simple math, I am an estimator by trade after all, I was able to determine that I’m getting robbed about five miles per hour or 3.75 miles on Strava (give or take), every time I throw a leg over my top tube on the trainer. In the scheme of things this doesn’t matter (some will say you’re not supposed to count indoor miles, anyway), but I had to know.

And, as one would expect, it sure was fun figuring all of that out.