Tire size

Better check your math. A 255/60/15 is 27” tall. A 275/60/15 is 28” tall.

That’s the diameter of course, but it means a 1/2” increase on the radius, so, it will raise the car about a 1/2” in the back. Assuming your speedometer was geared correctly for your old tires, your actual speed will be about 3.5% faster than indicated with the new ones. So for example if the speedo says 60 you’ll be doing ~62.1. The faster you go the larger the mph difference will be.