Are you taking a leisurely drive or going all out to get there? (Depending on if you've been down this route before or not?)
If you are taking your time, and wish to sight see along the way: You could take I-94 all the way over to Chicago and then retrace Route 66 from Chicago to Flagstaff AZ then take I-17 south to Tuscon. It may not be the most direct route but there are many stops along the way tracing Route 66 which can be quite fun.
(Its basically: I-55 to I-44 to I-40.)
Last summer we covered a small bit of Route 66 from Chicago to St. Louis. I think that was the most entertaining piece of the trip LOL
An example from our trip:
http://www.travelpod.com/travel-blog...0519/tpod.html