Dude, that sucks. I've been there before.
On another note, the ratings on spare tires is only for if you put said spare on a drive axle. The difference in outer diameter is what gets you. Your car's differential can only take so much slippage before it says, "I Quit". ...hence the milage and speed rating.
So you can jack up the side of your car that has the flat, put the good tire on the front and the spare on the back and drive till the tread wear down on the spare without any damage to anything, sin abs lights. Your car's comptater will be like' Too much spinny poo on tire!!' ....'initiate ABS light!!'