Route 66 is a common discussion topic among RVers.
The entire route doesn't exist anymore in any of its various configurations. Folks also have to remember that US-66 was always a changing highway. Many of the original segments were unsuitable for heavy traffic when the highway was officially created in 1926. Improvements and realignments were occurring from the start to the end of US-66.
The route of Henry Fonda and the Okies of the 30s was different from the route of the 50s and 60s. By that time the Kansas section had been bypassed, much of the Illinois original 66 had been moved to what is now I-55 and incorporated the freeway by 1957. In Oklahoma and Texas most of what remains is various 'Business Routes' through towns and cities of I-44 and I-40. There are several sections of US-66 through New Mexico - both alignments - the original route through Santa Fe and almost to Las Vegas, NM
, and the 1937+ alignment which now matches I-40 to bypass Santa Fe. Downtown Tucumcari, Grants and Gallup, NM
are my favorite parts of the old highway with a lot of old structures - even some 50's motels still open. In Arizona is a remaining section of 66 at the Painted Desert/ Petrified Forest NP
. You can see the old route which is closed to traffic along much of eastern I-40 in Arizona. There is a short segment from Winona through Flagstaff. The best preserved section is Seligman to Kingman, AZ
Probably 50% or so of the trip will have to be on other highways. Some of the most iconic segments which still exist are not open to automobile traffic any longer - like Chain of Rocks Bridge across the Mississippi.
Folks have made Wikipedia a very good, current, source of how to drive as much of the route as possible - and links to the various state preservation groups promote their highway.
The 'traditional' route is from Chicago to California. Many prefer that because it starts as typical Midwest flat land, farming communities and forests. As you go west the scenery gets more 'exciting' for many folks.
But it depends upon where you are locate and how much you want to travel to start the route. It is definitely a week to two week trip if you want to take time to explore the various preserved segments and tourist traps. After all the tourist traps where a big part of what made Rt-66 so fascinating.
Not all who wander are lost.