Dear Car Talk:
As I'm sure you are well aware, more and more cars today are sold without a spare tire. In many cases, buying a spare and throwing it in the trunk -- even a mini "donut" spare -- would take up a lot of room. And in the event of a high-speed accident, that could be dangerous in a car in which the cargo area is open to the passenger compartment, such as a wagon, hatchback or minivan, if the spare becomes an airborne projectile. Owners are told to make do with tire-repair kits, which include tire-repair fluid and compressors to refill the tire with air. That's fine for most punctures, but not all.
Here's my idea -- my question is whether this would be safe: Buy a mini donut spare tire, but only the tire, not the wheel. Choose a size that is big enough to fit around a flat tire (with the wheel jacked up), but small enough that after filling it with air from the repair kit compressor, it will fit tightly around the flattened tire. With the flat still mounted, fit the deflated donut tire around the flat. Then inflate the donut tire, which will tighten it around the flat. Then drive, slowly. What do you think? Sounds crazy, but it seems to me it would work and be safe. I think the main issue would be making sure to turn at a low speed if the flat is a front tire, so that the tire is not pushed off of the wheel.
Greg, it's thinking like this that has kept you out of some of the nation's top universities. The biggest problem with your idea is that tires are not soft. So it'd be almost impossible to "stretch" a tire over an existing tire. Sliding one tire over another is not like putting on two shirts. Tires have some flex in them, but they're not very malleable.
If you've ever had a flat tire and jacked up your car, you've probably noticed that without the weight of the car on it, the flat tire resumes its normal shape. It's not soft.
The other problem is that your existing flat tire will still be mounted to the wheel -- which means the inner bead of the tire will be pressed firmly up against the inside of the wheel rim. That'll make it impossible for you to seal that second tire over the first and get it to hold pressure.
So I think this is one of those great ideas that you probably shouldn't mention to anyone else. Not for fear that they'll steal it, but for fear that they'll call the guys with the white coats. Actually, there are lots of good ideas that are not workable at first, often for technological reasons. So this is my answer for now, Greg. But check back with me in 2030 when you can print a replacement zip-up tire from your spare 3D printer in the trunk.