The fair place to meet is El Paso, TX — the city closest to the midpoint of San Antonio and San Diego. From the farther side that’s about 13 hr 9 min of driving.
Recommended midpoint
El Paso, TX
From San Antonio
10 hr 31 min
502 mi to El Paso
From San Diego
13 hr 9 min
627 mi to El Paso
San Antonio has the shorter trip; the split is off by about 2 hr 38 min. The alternatives below can even it out. San Antonio and San Diego are about 1,126 miles apart.
San Antonio and San Diego are about 1,126 miles apart by road. Split the difference and you arrive near El Paso, the city closest to the halfway point between them. That puts roughly 10 hr 31 min of driving on the San Antonio side and 13 hr 9 min on the San Diego 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. El Paso still makes a fair, central place for San Antonio and San Diego to converge, splitting the travel instead of asking one side to cross the country.
If El Paso doesn't have what you're after, Tucson and Albuquerque are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between San Antonio and San Diego.
El Paso is a city in and the county seat of El Paso County, Texas, United States. It is the 22nd-most populous city in the U.S., sixth-most populous city in Texas, and the most populous city in West Texas with a population of 678,815 at the 2020 census. Read more →
San Antonio is a city in the U.S. state of Texas. It is the seventh-most populous city in the United States, the second-most populous city in Texas, and the second-most populous city in the Southern U.S., with a population of 1.43 million at the 2020 census. Read more →
San Diego is a city on the Pacific coast of Southern California, adjacent to the Mexico–United States border. It is the eighth-most populous city in the U.S. 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.