Nashville and San Francisco are about 1,959 miles apart by road. Split the difference and you arrive near Albuquerque, the city closest to the halfway point between them. That puts roughly 23 hr 25 min of driving on the Nashville side and 18 hr 46 min on the San Francisco side — the fairest single meeting point among the cities near the middle.
Over this distance most people will fly rather than drive the whole way. Albuquerque still makes a fair, central place for Nashville and San Francisco to converge, splitting the travel instead of asking one side to cross the country.
If Albuquerque doesn't have what you're after, Denver and El Paso are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Nashville and San Francisco.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.