Tire Rack study agrees with what I've found about Tread Depth

The old adage about Tire Tread Depth uses a penny and the top of Abe Lincoln's head to determine when your tread is gone.

This is about 2 mm of tread, and is in line with most safety laws and the 'wear bars' on tires.

My experience has been that tire performance degrades significantly long before that point and I haven't let it get that low in years. That's why I replaced the OEM tires at 40K miles even though I had about 4mm of tread left.

Now, Tire Rack has some objective test data to support this view.

Bottom line, in their opinion 4mm of tread depth is the minimum you should consider running before replacing the tires.

They point out that a Quarter using Washington's Head as the reference is about 4mm.

As far as I can tell, measuring tread depth was the only remaining practical use for a penny. Does this finally signal the end for Abe?