Saw a thread on a trucker forum about this exact thing. Guy said those cheap spares are basically rated for like 50 miles at low speed, so you got about what you paid for. They're called "temporary" for a reason, not "drive to the next state" tires. He said the rubber is thinner and they don't handle heat well at highway speed. Once that thing heats up on the interstate it's done for. You got 40 miles out of a tire that was meant to get you off the exit.