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Twelve days is actually quit a nice time frame for a one way drive from Philadelphia to San Diego by way of Chicago. One thing you should understand, though. US-66 was decommissioned over 20 years ago after it had been completely superceded by three Interstates: I-55, I-44 and I-40. They're not particularly romantic, nostalgic or idyllic, and trying to follow the old US-66 asphalt, while technically feasible, would be a navigational challenge and a big time sink. I think you'd do far better to use I-80/I-76 to the Denver area and see Rocky Mountain National Park, then take I-70 (one of the most scenic Interstates of all) through the Rockies to US-191 south to Arches National Park and on to US-163 through Monument Valley to the Grand Canyon. From there I-40 and US-93 would take you to Las Vegas and I-15 would complete your journey to San Diego

AZBuck