How does your tire choice come into play when you experience oversteer or understeer? By matching your tires, you'll maintain the same overall tire grip in the front and back. When you have mismatched tires, you're either adding more grip to the front or more grip to the rear that causes abnormal and/or unpredictable oversteer or understeer. We've literally tested hundreds of tires and there truly are differences in wet, dry and snow grip.
You don't have to be driving fast for a tire mismatch to create problems. For example, if someone abruptly stops in front of you to make a left turn and you brake hard, cutting sharply to the right to avoid him, your mismatched tires that have more grip in the rear, may push your car too far to the right so that you end up over correcting to the left. If you drive by an accident scene you may see skid marks in one direction and skid marks in another direction documenting this scenario. Modern cars are designed to be easier to drive and traction control helps tremendously, yet drivers still lose control of their cars. There's no point in creating an unnecessary risk by mismatching tires.