I have a 2007 mustang gt with 18 inch factory rims. Can I mount 275/40/18 tires on these rims in the rear.
Depends on the width of the rim. The tire will give you required rim sizes. If it fits, then it’s up to you.
If you’re changing the overall height of the tire, then your speedometer will also be incorrect. Changing to differing sizes (front to rear) will also have an impact on ABS and traction control.
You must maintain the same CIRCUMFERENCE front and rear or your ABS will malfunction and not work…
I would not change tire sizes unless the overall diameter remained the same as factory spec. and it was done to all four wheels. The car was not designed to safely accommodate different sized tires front to back. That’s reserved for the big boys whose handling geometry is quite different.