I just did it in June on my Harley. Took 66 from Chicago out to Santa Monica, then up the #1 Pacific Trail to Frisco, then cut back along the Lincoln (better known as the Pony Express). The further east you go, the better the Lincoln gets, especially starting in Illinois. The section through Lake Tahoe Valley is very nice, well developed and well travelled. Much of the Lincoln is actually Interstate 80 now. The two-lane "loneliest highway" bit is not that lonely compared to some Canadian highways, but it's nice to stop and read the roadside info signs, even though much of the highway is rather boring.
Most of the original sections of 66 and the Lincoln are not drivable. They have actually built a new highway next to each of them almost everywhere. You can however, see or even access the original but unmaintained highways in a few places. It's interesting to see abandoned bridges in the middle of the forest in Missouri, and other areas. Of the two highways, the Lincoln is far better maintained, especially U.S. Rte 50 (the original Pony Express) from California through Nevada and Utah.
Whatever you do, don't skip the Oatman Highway. It is an original part of old 66 and a genuine treat on a motorcycle. Also, the Santa Fe Trail is the way to go too.
To be realistic, you can find old highways like these in Canada too. It's just that the Americans are so good at marketing that people tend to believe it. There are places in the American west I remember from old Hollywood movies, abandoned truck stops and that unique western flavor that evokes images of a lost America, circa the 40s, 50s, 60s and 70s. Much of 66 has vanished since I last rode it in 1989 and it continues to disappear, including the famous land marks.
A tip to finding the original highways is to look for the power lines. While roads are moved, Hydro companies rarely move their lines from the original route and they clear the original road for crews to maintain them. This is especially true with the Lincoln. Here is a pic of the original highway from my trip:
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