I just bought some fix-a-flat the other day, and on the back it said "warning: do not use in z-rated tires." Why is this? The only thing I can think is that they don't want to be liable for a high speed blow-out.

 

You shouldn't use that stuff at all really... it makes it a serious pain when your tires are dismounted for repair or replacement. Just use the donut spare and take it to get the leak fixed or a new tire.

 

In most cases that is what I would do, but I just got for an emergency.

 

They're probably concerned about the fact that using Fix-A-Flat throws your tire a little out of balance, especially if you use it wrong. At high speeds it can cause serious issues.