The fair place to meet is Indianapolis, IN — the city closest to the midpoint of Milwaukee and Nashville. From the farther side that’s about 5 hr 16 min of driving.
Recommended midpoint
Indianapolis, IN
From Milwaukee
5 hr 6 min
243 mi to Indianapolis
From Nashville
5 hr 16 min
251 mi to Indianapolis
That’s an even split — both sides drive within 10 min of each other. Milwaukee and Nashville are about 479 miles apart.
Milwaukee and Nashville are about 479 miles apart by road. Split the difference and you arrive near Indianapolis, the city closest to the halfway point between them. That splits the trip almost evenly — about 5 hr 6 min from Milwaukee and 5 hr 16 min from Nashville.
That's a half-day drive from each side, so Indianapolis suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Indianapolis doesn't have what you're after, Louisville and Cincinnati are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Milwaukee and Nashville.
Indianapolis, colloquially known as Indy, is the capital and most populous city of the U.S. state of Indiana and the county seat of Marion County. Indianapolis is situated in the state's central till plain region along the west fork of the White River. Read more →
Milwaukee is the most populous city in the U.S. state of Wisconsin. It is located on the western shore of Lake Michigan at the confluence of the Milwaukee, Menomonee, and Kinnickinnic Rivers. Read more →
Nashville is the capital and most populous city in the U.S. state of Tennessee. It is the seat of Davidson County in Middle Tennessee, located on the Cumberland River. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.