There are several pictures on the Internet of old tires with the tread separated from the sidewall, or where the sidewall itself simply cracked all the way through. And sometimes that happens with the car just sitting in the garage! Doesn't matter how much tread the tire has, or how many miles are on it. I wouldn't take a chance at any speed, or you could find yourself with one wheel on the pavement and what's left of the tire flopping against the inner fender. I cut this from a classic car website: "It might be heartbreaking to throw away what looks like a perfectly good set of Pirellis or Goodyear Eagles, but consider the fact that tires are made of rubber and rubber rots over time, like an old rubber band that cracks and breaks when you try to stretch it. This is one of the facts of life when it comes to classic cars and other vehicles that are infrequently driven, such as motorhomes and campers, and that old pickup truck you leave up at the cabin. Time and dry rot are tire killers that can’t be avoided, and while the tires might not have too many miles on them, they still need to go."