I have been looking at a unique opportunity to determine how fast my thumbnail grows. When I got in my car wreck last December, the depoying air bag's cover hit my thumbnail, and gave me a small bruise. I suppose it was coincidence that it happened to line up almost exactly with the edge of my lunula. Since then, it has been traveling up my thumbnail, more or less intact.
Carefully measuring its position, I can see that it has traveled about 6/16 of an inch since December 11, which is 70 days ago. Some quick calculations indicates that this translates to almost 2 inches (5 centimeters) a year. This is 0.0000000035 mph, in case you were wondering.
Searching on the web, I can find growth rates between 0.8 and 1.8 inches per year, so that puts me right on the edge of the upper bounds. If those figures are accurate, perhaps my thumbnail just grows faster than most, or maybe my thumbnail injury did not move along precisely with my thumbnail growth as I'm assuming.