This is a good discussion and offers some education for all of us. I am not any kind of expert, but I have tried to learn enough to be safe and responsible when it comes to tires.
In the end, this is a judgement call, based on lots of factors, but we are responsible for consequences. I have lots of tires on different kinds of equipment on the farm. Do we "repurpose" old tires for use on our wagons? Of course we do, and many tires are cracked (not really a good measure of dependability by the way), balding, and 10-20 years old. But they don't really go off the farm...
An old Chevy dual wheel truck has tires with plenty of tread, but I only trust it to go on very short trips locally. Mechanically sound, but I don't allow anyone to drive it but me. Because I do know what might happen. A risk, but I accept it and prepare for it.
Folks, I have had tires correctly sized/rated with plenty of tread, less than 5 years old, self-destruct at highway speeds. Results are impressive - steel channel bed supports bent and twisted from impact and pieces of shrapnel flying everywhere. I have helped repair trailers where a tire let go and a steel radial "whip" destroyed lots of stuff...lots.
So when my one-ton Duramax leaves this farm with a heavy load behind it, it rides on the best Michelin tires I can put on it, properly inflated, and never more than 5 years old.
Why, when I could go cheaper? Well, put yourself in the pilot's seat: ambient temps are 95+ degrees and you're adding to the tire's temps at road speed with load and friction. You're heading down the road, cup of coffee next to you, good music...and a tire blows/delaminates. Now, picture in oncoming traffic an SUV coming toward you with your wife, or children, or grandchildren in it. Best way I can put it.
