I want to get winter tires (preferrably 17" to keep the cost down) for my 2007 Cadillac SRX Sport which has 20" tires (255/50ZR20). I note that the outside diameter for them is 30.1" yet both the V6 with 17" and V8 with 18" use tires with an outside diameter of 29.1 ". It appears the Sport package speedometer must be calibrated differently since there is a 1" diameter/3" circumference difference! Otherwise the speedo would be out by about 3 MPH (5 KPH). Can anyone confirm this and recommend a 17" tire size equivalent that would work to not bugger up the speedometer? Perhaps the front tires from the V6 235/65R17 (afterall narrow is better) and just take a small hit on speedometer accuracy.