A wear indicator stencil. It's shaped to a new tyre, you pop it on the tyre and the gap shows wear. Boom, copyright JH! [emoji6]
Or different colour rubber for the last bit of tread so when it shows through you know it's time for a change, copyright NZDave .
Damn, I've already sold the house and started production of my wear stencil. That's a much neater solution. I'll just market them as 'smiles' to hold in front of your face... Or fake Fu Man Chu style moustaches