Here's a route Toronto to Austin that actually avoids Chicago and St Louis. Great cities, but traffic nightmares.
Toronto to Detroit: CA 403, CA 401 (230 miles)
Detroit to Cincinatti: South on I-75 (260 miles)
Cincinatti to Louisville: SW on I-71 (100 miles)
Louisville to Nashville: South on I-65 (175 miles)
Nasville to Memphis: SW on I-40 (215 miles)
Memphis to Little Rock: SW on I-40 (140 miles)
Little Rock to Dallas: SW on I-30 (325 miles)
Dallas to Austin TX: South on I-35 (200 miles)
Driving distance: 1629.9 miles
Driving time: 26 hours, 31 minutes