A little background. I bought my current car in July 2011. I have no clue what kind of tires were on it then, only that they didn't have a whole lot of tread left. I was told when I bought the car that I'd need to replace all four tires within a year or so. So, in December 2011, I was told that I should replace the tires. I didn't have a whole lot of money at the time, so I elected to replace the two rear tires since they were worse - again, I don't know what the original tires were, but I replaced them with Coopers as they were having some sort of sale/deal. In July 2012, I had the front tires replaced since they were now pretty bad - they put Firestones on. Now I'm hearing a lot that it's bad to have mismatched tires - that for ideal driving and handling, all four tires should be the same, and that may account for the vibrations I'm feeling at highway speeds. Is that true? If so, is it possible to just buy two new Firestones for the back? Or would I need to buy four brand-new tires and "start fresh" so to speak? The last time I got an oil change (in October) the treads on the rears measured 9/32 and the treads on the fronts measured 11/32, if that matters.