If you're serious about the trip, invest a few dollars in the EZ66 Guide which Steve offers in the "Shop" section.
If you merely want to sample a few places, these Interstates roughly parallel Route 66: I-55 (Chicago to St Louis); I-44 (St Louis to Oklahoma City); I-40 (Oklahoma City to Barstow CA); I-15 (Barstow to San Bernardino CA). In some cases, Route 66 is an Interstate frontage road or just a short distance away.
But with all the big trucks and bad pavement, Chicago to LA on the Interstates is punishing--even in a nice car.
By contrast, Route 66 has many stretches of original pavement (or at least original alignment) that make for an enjoyable ride, especially on a motorcycle.