The fair place to meet is Phoenix, AZ — the city closest to the midpoint of Denver and San Diego. From the farther side that’s about 12 hr 17 min of driving.
Recommended midpoint
Phoenix, AZ
From Denver
12 hr 17 min
586 mi to Phoenix
From San Diego
6 hr 16 min
299 mi to Phoenix
San Diego has the shorter trip; the split is off by about 6 hr 1 min. The alternatives below can even it out. Denver and San Diego are about 833 miles apart.
Denver and San Diego are about 833 miles apart by road. Split the difference and you arrive near Phoenix, the city closest to the halfway point between them. That puts roughly 12 hr 17 min of driving on the Denver side and 6 hr 16 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. Phoenix still makes a fair, central place for Denver and San Diego to converge, splitting the travel instead of asking one side to cross the country.
If Phoenix doesn't have what you're after, Las Vegas and Albuquerque are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Denver and San Diego.
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 →
Denver is the capital and most populous city of the U.S. state of Colorado. Officially a consolidated city and county, it is located in the South Platte River valley on the western edge of the High Plains, and is just east of the Front Range of the Rocky… 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.