I don't have Coopers. I do have Pirelli Scorpion All Terrain Plus 275/60R20. I felt the need to calibrate. I was off 2 mph at 50 and 3 mph off at 60. The MPH difference wasn't that bad; but there are other things it affects. First, the transmission shifting seemed to change...and not for the better. Second, the MPG calculations dropped by 2.5 MPG. I bought the Rough Country calibrator for $139 and it took about 15 minutes to get everything right. Now, my speed is dead on with GPS, the transmission shifts normally again, and I am getting accurate MPG calculations again. The $139 for calibrator and the little bit of time it took to do it are well worth it.