The fair place to meet is Las Vegas, NV — the city closest to the midpoint of Phoenix and San Diego. From the farther side that’s about 5 hr 33 min of driving.
Recommended midpoint
Las Vegas, NV
From Phoenix
5 hr 22 min
256 mi to Las Vegas
From San Diego
5 hr 33 min
265 mi to Las Vegas
That’s an even split — both sides drive within 11 min of each other. Phoenix and San Diego are about 299 miles apart.
Phoenix and San Diego are about 299 miles apart by road. Split the difference and you arrive near Las Vegas, the city closest to the halfway point between them. That splits the trip almost evenly — about 5 hr 22 min from Phoenix and 5 hr 33 min from San Diego.
At this range, Las Vegas works for a same-day meetup: close enough from both Phoenix and San Diego to meet for lunch or an afternoon and still be home by evening.
If Las Vegas doesn't have what you're after, Los Angeles and Tucson are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Phoenix and San Diego.
Las Vegas, colloquially shortened to Vegas, is the most populous city in the U.S. state of Nevada and the county seat of Clark County. It is the 24th-most populous city in the United States, with 641,903 residents at the 2020 census. Read more →
Phoenix is the capital and most populous city of the U.S. state of Arizona. With over 1.6 million residents at the 2020 census, Phoenix is the fifth-most populous city in the United States and the most populous state capital. 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.