The fair place to meet is Phoenix, AZ — the city closest to the midpoint of San Diego and Tucson. From the farther side that’s about 6 hr 16 min of driving.
Recommended midpoint
Phoenix, AZ
From San Diego
6 hr 16 min
299 mi to Phoenix
From Tucson
2 hr 39 min
106 mi to Phoenix
Tucson has the shorter trip; the split is off by about 3 hr 37 min. The alternatives below can even it out. San Diego and Tucson are about 362 miles apart.
San Diego and Tucson are about 362 miles apart by road. Split the difference and you arrive near Phoenix, the city closest to the halfway point between them. That puts roughly 6 hr 16 min of driving on the San Diego side and 2 hr 39 min on the Tucson side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Phoenix suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Phoenix doesn't have what you're after, Las Vegas and Los Angeles are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between San Diego and Tucson.
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 →
Tucson is the county seat of and the most populated city in Pima County, Arizona, United States. 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.