Today I replaced a 'squared off" Gator HardShell with close to 2300 miles with a new one. Didn't figure that another 15%-20% of mileage would be worth an episode or two of standing beside the road while replacing a tube.
Using a micrometer I measured the thickness of the old and new tire and found that the old tire is .102" thick at the contact point and the new is .144. Assuming typical Germanic consistency (Continental Tire), the tire was mostly clapped out with a tread wear of 42 thousandths of an inch (.042"). And that's a relatively heavy tire!
Not much tread on these tires. And they say to replace your car tire when it gets to the thickness of a penny. That's about triple the tread depth of a new bike tire.