Tire Size worth worrying about?

[Skip_1074]

Installed my CTS today, and all went smooth. The hardest part was removing/reinstalling the trim panel on the side of the dash and routing the OBD II cable.

So my truck came stock with P265/70R17 Michelin LTX A/T2 tires (31.61 inches using basic math, 657 RPM). When I was programming the truck, I checked the tire size and it said 30.9 inches. The only size that I can come up with that is close to that height is a 255/70R17 (31.06 inches using basic math, 669 RPM for comparable tire Michelin tire). I calculate this at 1.7-1.8% difference.

What are your opinions on recalibrating for the difference. I know that amount seems negligible, but I am avid about tracking MPG (I know its a truck, but I still track it) and just making that small change to the mileage calculations I already have, that would be a 0.237 MPG increase over the stock setting.

I don't have the capacity to do a rollover test on the tires or GPS it (well I don't trust the accuracy of my iPhone GPS), what are your inputs?

[Jackpine]

Please check my response in the other thread: http://forum.gopowerhungry.com/gryph...html#post43220

- Jack