Okay, ready for some super duper geekery?
* First, make a big, obvious mark on the treadmill belt. I used a hardware store paint marker;
* Next, set the treadmill to a speed and a grade that you can easily maintain while thinking about other things;
* Next, start walking on the treadmill with a stopwatch, focusing on the part of the belt that you put the mark on;
* When the spot first appears, start the stopwatch;
* Continue walking on the belt, counting how many times you see the mark;
* Stop the stopwatch after 100 revolutions.
Does it have to be 100 revolutions? No, but the more revolutions you use, the more accurate the calibration will be.
Do you have to walk on the treadmill? No, but if you don’t trust your treadmill, then I would make it as “real life” as possible.
If you have an x11i, then I’m pretty sure the belt is 128″ long. 100 revolutions is 0.2020202 miles. To get miles per minute, divide the time for 100 revolutions by the duration. Then multiple the result by 60 to get miles per hour.
For example, if it took 5:20 for 100 revolutions, then:
128″ * 100 = 12,800″ = 0.2020202 miles
0.2020202 / 5.3333′ = 0.3787879 miles/min
0.3787878 * 60 = 2.27 mph
If you’re using kmh, then use 0.32512 instead of 0.2020202.
Lastly, divide the result by the displayed speed of your treadmill. If your treadmill is set to 2 mph for the above, then it’s running 13.5% too fast:
2.27 / 2 = 1.135
1.135 – 1 = 0.135
0.135 * 100 = 13.5%
Hopefully that makes sense. If not, or if I made any mistakes, please let me know.
If you’re training by heart rate, the speed of the treadmill won’t matter at all. But if you want to train by pace (either horizontal or vertical), then I recommend knowing the speed. I was being very specific with high intensities, varying them by only a few percentage points when at or above AnT, so I wanted to make sure that I was doing what I intended. (Going too hard unintentionally would have been a disaster.)
(As an aside, I first discovered this by going to a gym with a better treadmill. I set the speed for my usual warm up and it was super easy. I was confused for a while until i figured out what was going on.)
With mine in particular, the steeper I set the trainer, the faster it runs above the displayed speed.