Another vote for the EZ66 Guide for Travelers.
At one time or another over the past 35 years, via car and motorhome, I've explored most of Route 66 from the outskirts of Chicago to Flagstaff, but have not taken it into southern California. Route 66 is closely followed by I-55, I-44 then I-40 in succession (except a large variation between Springfield, Missouri and Fairfield, Oklahoma where the Interstate totally misses the Kansas section of 66).
With a good guide, you can find the places you might want to drive the old road, and others where you'll want to hop onto the Interstate to make some progress. In particular, you'll probably want to take expressways through or around major cities like St Louis, Tulsa and the northern suburbs of Oklahoma City, rather then being hours in stop and go traffic on city streets.
Even in rural areas, travel on 66 is going to take extra time. For example, Tulsa to Oklahoma City is usually 1 1/2 hours on I-44 (in a car, at the speed limit). On Oklahoma 66, that's about a three hour drive, or avoiding the worst urban sections, 2 hours 45 minutes from Sapulpa to Arcadia.
Even so, for many years I would get on 66 at Sapulpa and take it to Chandler if going to Shawnee, or to Luther, if going to Norman. This plan would usually include a meal stop at a restaurant or diner in Chandler, thus not in a hurry.
Similarly, Oklahoma City to Amarillo is a bit under four hours using I-40. Last time I followed Route 66 in touring mode, I took two and a half days getting from El Reno to Amarillo. That one was a RV tour, and I diverted from 66 to get to Clovis and SE New Mexico destinations.
Another section, Joplin to St Louis, is four hours by I-44. Driving on 66 and stopping at the places that interested me, it is at least eight hours, and if you hit all the old Route 66 tourist attractions (most are still there) and winery tours, two or three days to cross Missouri.
So if you have a fixed destination at a fixed time, and want to do a Route 66 tour on the way, you'll need to do some homework and plan that tour in detail, deciding on stops and allowing time for those stops, so you can get to your destination on schedule.
A young friend of mine from China recently finished a Route 66 trip (by car) and used ten days to get from Chicago Art Institute to the Santa Monica Pier. From the photos she sent, they did not use the Interstates very much, visiting a lot of the urban locations.
Tom Test
Itasca Spirit 29B