The fair place to meet is Columbus, OH — the city closest to the midpoint of Cincinnati and Detroit. From the farther side that’s about 3 hr 26 min of driving.
Recommended midpoint
Columbus, OH
From Cincinnati
2 hr 30 min
100 mi to Columbus
From Detroit
3 hr 26 min
164 mi to Columbus
Cincinnati has the shorter trip; the split is off by about 56 min. The alternatives below can even it out. Cincinnati and Detroit are about 236 miles apart.
Cincinnati and Detroit are about 236 miles apart by road. Split the difference and you arrive near Columbus, the city closest to the halfway point between them. That puts roughly 2 hr 30 min of driving on the Cincinnati side and 3 hr 26 min on the Detroit side — the fairest single meeting point among the cities near the middle.
At this range, Columbus works for a same-day meetup: close enough from both Cincinnati and Detroit to meet for lunch or an afternoon and still be home by evening.
If Columbus doesn't have what you're after, Cleveland and Indianapolis are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Cincinnati and Detroit.
Columbus is the capital and most populous city of the U.S. state of Ohio. With a population of 905,748 at the 2020 census, it is the 14th-most populous city in the U.S., second-most populous city in the Midwest, and third-most populous U.S. state capital. Read more →
Cincinnati is the most populous city in Hamilton County, Ohio, United States, and its county seat. Read more →
Detroit is the most populous city in the U.S. state of Michigan. It is situated on the bank of the Detroit River across from the Canadian city of Windsor, Ontario. It is the 26th-most populous city in the United States and the largest 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.