The calculated distance (air line) between Castelnovo ne'Monti and San Martino in Rio is approximately 28 miles respectively 45 kilometers.
By car or train, the actual journey to San Martino in Rio is certainly longer, as only the direct route (as the crow flies) between Castelnovo ne'Monti and San Martino in Rio has been calculated here.
Air line (approximately)
45 kilometers
24 nautical miles