This is my current wheels, 18 x 9.5 with 265/40/18's on the rear, not sure on the backspace though, their website lists them as available in 6.75" , 5.75" or 5.25" so i'll have to take one off and check

With the current wheel off any idea what measurements I need to do in the arch to see if they will fit ? So just to clarify I want the 15 x 8 with 275/60/15 tyres and 3.75" backspace, I assume I should just measure from the hub to the wheel arch lip then find out the exact width of the tyre ? Thanks