The smaller tire is about 2.3% smaller in diameter so your speedo will have a 2.3% error; will read high as will your odo when you have the smaller tires on the drive wheels. Verify this by Googling Tire Size Calculator.
I ran 155s and 165s on a car until the wrong size wore out; I couldn’t notice the difference.
As was said, rotate but make sure that you keep the same size on the front and the same size on the rear.