Many years ago, I used to drive a Signal Yellow Ford Fiesta 1.1S (all 3 of my cars have been bright yellow - it's a very safe colour). In 1982, around Christmas time, I had a minor prang which dented the front offside just beside the headlamp. When I got home, I tried to get my hand up the front wheel arch to inspect the damage but the wheel was in the way so I applied full left lock. What I then saw sent a chill down my spine.
Both front tyres had perfect tread on the outside edges but were worn right down to the steel belts on the inside edges. This was due to tracking error (excessive toe-out) forcing the treads outward as the car moved forward. When I had the tyres replaced and the tracking corrected, the steering felt much lighter (I didn't have power steering).
Fast-forward to 30th December 2009....
I had a slow puncture in my rear nearside tyre (due to a nail) so I went to the garage to get it repaired. The garage informed me that both rear tyres were unserviceable due to a complete lack of tread between the inside & outside edges of the tyres. My car has wide tyres and apparently, this happens even when tyres are inflated to the correct pressure.
Last week, I was driving around on snow & ice on two semi-bald tyres that provide traction, as Mazda MX-5s have Rear Wheel Drive. I got lucky...twice!
Moral of the story:- Check your treads across the full width of the tyres.