My 17" spare has a tire that matches the diameter of my 19" sport wheels and is rated at 130 mph. I've always felt bad for those cars seen limping home on their undersized spares. Having a speed rated spare tire was very important to me when choosing a car or SUV. As an example, he Porsche Cayenne has a serious spare problem. I'm thinking run-flats will become more popular as they improve the technology.