Before I go any further, let me make it clear that we do not, under any circumstance condone tire stretching; it diminishes vehicle handling and can be downright dangerous depending on the extent of the stretch. However, if stance is your life and your heart aches for that hellaflush look, here’s how tire stretching is done.
First, a generous amount of lighter fluid is sprayed along the sidewall, with the tire mounted on the wheel.
Then, some air is added, the sidewall is lit on fire and bam! The tire is mounted. Fill it with air and you’re ready to roll on your undersized rubber. The idea is that as the fire burns, it draws oxygen out of the tire, forcing it against the wheel. Here’s a video by some clever German’s demonstrating how it’s done.