If I remember right, they are 3.75 or 4 inch. I was reading on here which backspace to go with but everyone seemed to have a different one so I picked the 3.75 im sure. BUT with my Drag Radials, they did rub the top of the frame on the inside wheel well. So I bought a 1/4" spacer and that took care of it. And that was with the stock springs and shocks. The new shocks look as if they were stock replacements and they work flawless. I set my fronts to 90/10 and rear to 50/50. I put my springs on at the same time I did the springs, so I think without the spacer it still would rub. So a 4 inch backspace would have been better. Hope that makes sence lol
Pics of my 455 Regal. Eibachs on all four corners. 275/60-15 recap stock car tires on stock GN steels on the back. 16" WS6 snowflakes with 225/50-16 rubber on the front. That pic is deceiving. I'm flying over a dip in the course so the front end is at the limit of the suspension. Normal stance it sits about 2-3" lower on the front. Slight rub on the rear rail. You probally wouldn't have a problem with a standard sized 275.