My favorite coincidence in this vein involves feet, nanoseconds, and the speed of light. In special relativity, physicists typically work in "geometric units", where <i>c</i> = 1. Measuring length in feet and time in nanoseconds yields<p><pre><code>  c = 0.983571056 feet/nanosecond
</code></pre>
This is close enough to 1 for most practical purposes.