Found this interesting item in the Morning Call, a newspaper serving the Lehigh Valley area in Pennsylvania. The response was written by a local mechanical engineer:
Q: Recently my car failed the Pennsylvania inspection because it had a doughnut spare tire installed. I was told that all tires on a car have to be the same size. Why can this type of spare tire be sold with the vehicle but not be good enough to pass inspection?
A: The ''doughnut,'' also called a compact spare or mini-spare, is a ''limited use'' tire. It is smaller in diameter and narrower than a full-size spare, intended for temporary use (typically 50 miles and 50 mph). Doughnut tires came into use in the late 1970s and early 1980s to cut cost, reduce vehicle weight for fuel efficiency and save space.
Pennsylvania's inspection is supposed to make sure that a car is safe for one year. So a tire good for only 50 miles doesn't pass muster.
Also, Pennsylvania Code 167 chapter 175.65 (d) states: ''Tires of different types, such as one snow tire and one regular tire or bias belted or radial tire, may not be used on the same axle except in an emergency.''
The doughnut spare is safe to use as long as you adhere to the speed and distance limitations marked on it. You should replace the doughnut with a proper full-size tire immediately not next week or when you get around to it, but at the next service station. If you use a significant amount of the 50 miles, you should replace the doughnut with a new one.
Good engineering practice is to design a product not just for its intended use, but also with consideration for how ''reasonable people'' might use the product. In my opinion, it is not a stretch that the 50 miles/50 mph limitation could be exceeded. But you would not be as safe as with a full-size tire. I found no accident data online concerning accidents with doughnut tires (or full-size tires for that matter). I assume most people realize the doughnut is not as good and drive appropriately.
The trend is for fewer spares. Reasons include improvements in tire quality, roads, infrastructure (service stations, tow trucks) and communication (cell phones). In the early days, when driving was an adventure, it was not uncommon to have three spare tires (without wheels) as blowouts were common. Later keeping two spares behind the front fenders was common.
Today, flats are relatively rare events and it's not uncommon to see a vehicle make it to the salvage yard with the doughnut spare having never been used! Some new cars come just with a pressured bottle meant to fix a flat.
In the future, perhaps, repair crews will respond to your cell phone's GPS signal, eliminating the need for crawling around outside your vehicle after giving directions.
Whether you have a doughnut, full spare or bottle of air, what's important is everybody who drives the vehicle knows all about it.