Miami and San Francisco are about 2,591 miles apart by road. Split the difference and you arrive near San Antonio, the city closest to the halfway point between them. That puts roughly 24 hr 4 min of driving on the Miami side and 31 hr 11 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. San Antonio still makes a fair, central place for Miami and San Francisco to converge, splitting the travel instead of asking one side to cross the country.
If San Antonio doesn't have what you're after, Austin and Dallas are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Miami and San Francisco.
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.