So I assume that your starting your trip in Chicago? (Rt 66) How many days are you planning on the road???
We spent 28 days in Sept 2010----but did add return trip from up north going to Yellostone,Grand Tetons and Mt Rushmore---Sept was a good month as the temps were nice (not too hot in Arizona and just at freezing overnite camping out at Yellowstone) traffic was't bad (kids all back in school,vacations for many were over)and all the sites still open---------