Iwant to change my stock wheels 225 55 R16 to: 17x8 rims 6" backspace 235 55 17

The best thing to do is determine outside diameter of the tire and wheel with each scenario. The 16 inch stock tires and wheels have an overall outside diameter of 653.9 millimeters. The 17 inch proposed tires and wheels have an overall outside diameter of 690.3 millimeters. This is not an acceptable swap. If you want to put 17 inch wheels on your Mustang, the aspect ratio (the "55" number) will have to be smaller. The correct swap to be as close as possible would be a 245-45-17 tire on your 17 inch wheels. I would suggest working with one of the many tire and wheel suppliers available to you for guidance, such as Tire Rack - (www.tirerack.com) or Discount Tire (www.discounttire.com). The reason this is important - the powertrain control module and antilock brake control modules need to have an accurate outside diameter of the wheel and tire matching module programming to provide proper operation of the vehicle, and proper speedometer output. This along with the "clearances" with steering and suspension in mind. Plainly stated, the 235-55-17 tires are far too tall to be used.