Mike, there is a cycling map put out by the Adventure Cycling Association that is called Route 66 cycling map. I cycled part of the old highway 66 when I cycled from Amarillo, Texas to New Brunswick, Canada in 2019. There aren't a lot of original segments of route 66. We were on some parts when they were the frontage roads along I-40. No shoulder, two lanes, a little shallow curb to direct rain water. It's amazing that it was the main highway from Chicago to Santa Monica. There are some great sights along the way. Shamrock, Texas stood out to me.
John