The whole deal with Fix-a-Flat is that it works well in tires that can be inflated to a low PSI and still driven.
The liquid has some ability to plug small holes, but of the air pressure is too high, the air will blow the f-a-f right through the hole. My motorcycle customers sometimes try f-a-f to get back onto the road, almost universally without success. The reason is that a car tire can be driven on with 15 psi inflation, but a motorcycle tire usually requires 35 to 50 psi, far more pressure that will blow the liquid through the hole it is trying to block. Also, the stuff will throw your tires out of balance, and it will prevent you from avoiding a larger disaster down the road. With f-a-f in your tire, if you pick up a nail, or a jigsaw blade or something large, the f-a-f may seal the puncture for a while, allowing you to continue driving along, but if that object that has compromised the integrity of your tire suddenly catches on something, shifts position or whatever, it may rip the already weakened tire wide open without warning, causing sudden deflation and loss of control. I am firmly against Fix-a-Flat in vehicle tires, but it works great for things like wheelbarrows, garden tractors and golf carts, etc.