Don't take a corner with them. Stretching is definately not the best of ideas to do when it comes to tires. The sidewalls are designed as a structural member with the rim and is basicly compromised when one is foolish enough to put smaller tires on wide rims. A tire that's too narrow under hard driving has the tendecy to come unbeaded and, well, let's just say you'll probably have a bad day after that. Upgrade to the proper sized tire. At least a 255 series or be even smarter and just reduce the rim diameter and make a compromise for an 18 inch rim.