The "Mother Road" as John Steinbeck called it ...
... was the most important east to west thoroughfare carrying the traffic all over the continent from Chicago, IL to Los Angeles, CA. Travelling the old highway, which was comissioned to be "US66" from 1926 to 1985, modern roadies will cross 8 states (IL, MO, KS, OK, TX, NM, AZ, CA) and 3 timezones. Everybody interested in history has to look out for mile of mile deteriorating, sometimes forgotten roadbeds beneath desert sand, sagebrush or (that damned) interstate, but there are also hundreds of contigouos miles still alive and drivable (some of it with proper equipment only - be careful!).
1925 planning of US60 (later US66)
1927 description of US66
About 80% of the historic US66 is still existent. Many stretches of what remained are still accessible, most of it by bike or car (maybe 4 wheel recommended !)
There is lot of contiguous road all along the original 2448 miles between Chicago and Los Angeles you can drive thru. AZ claims the sector between Ashfork and Topock as the longest contiguous stretch of remaining US66 - but that statement is well discussable.
Here are all segments of US66 you can drive uninterrupted (that means without gaps of more than one mile like Interstate ramps, short detours etc.) on the Mother Road as it presented itself not later than in the 1940s. Most of it are from the original 1926 alignment. Please refer to the atlas and milepost sections for an exact routing.
I only picked up those which have more than 50 miles.
308 miles C(h)affeyville,MO to Depew,OK
274 miles Joliet Road,IL (I-55 Xit269) to East St. Louis,IL
173 miles Luther,OK to Sayre,OK
164 miles Goffs Road,CA (I-40 Xit Las Vegas) to W of Daggett,CA
158 miles W of Ashfork,AZ to Topock,AZ
60 miles Mesita,NM to Thoreau,NM
60 miles Devore,CA to Pasadena,CA
56 miles St. Louis,MO to St. Clair,MO
52 miles W of Barstow,CA to Hesperia,CA
51 miles Algodones,NM to Los Lunas,NM