bobfil, just to make sure we are keeping things straight here... you initially said "255x75x15" tires. Then someone else chimed in about 275x60x15.
The first number is the width. The second number is the sidewall height (sometims referred to as the 'series' number.) The third number is the rim size.
You don't want 75 series tires on your car. From the factory, at most they came with a 70 series, such as a 225x70x15. This is not a performance size tire.
There's also 275x__x15 tires, but those won't fit inside your wheelwell.
I'd suggest no more than a 255x60x15 in the rear. That's what I have on my car, and I got them from TireRack. However, check locally for the same tire. After you pay for shipping and mounting, sometimes it's just cheaper to get it all taken care of locally.
If you just burble around in your car, you could go to a slightly slimmer tire for slightly better tracking and gas mileage, such as a 225 width.