Thanx, I'll have to save more $$$$ and change all four.

Apart from mixing tires which will give somewhat unpredictable results, the stability control system takes each tire's traction into account, as it would if all the tires were the same but on differing surfaces (like combinations of gravel, wet, ice and tar).

However, mixing tires means that when you are pushing the limits and by necessity you turn off one or more of the management capabilities, differing tires will create unpredicatable results.

If the car is designed for the same tires for all four corners you should heed the engineer/designers principles. In some cars like Ferraris and Lasmbos, you might see different width tires, but not normaly differing compound or brands of tires.