It's actually part of how I know how fast I was going that one time. :-) I was on a 45mph speed limit road and I was keeping up with the cars, but I wasn't sur

It's actually part of how I know how fast I was going that one time. :-) I was on a 45mph speed limit road and I was keeping up with the cars, but I wasn't sure if they were following the limit. But I was later able to tap at the same frequency while looking at a watch and discovered it was about 8 Hz, and then took the outer circumference of my tire and calculated that it was about 45 mph as well assuming that an imbalance in the wheel or tire was the source. (My video camera had also caught some mile markers, but my speed varied in between, so those were only corroborative.)