The calculated distance (air line) between Terni and Guidonia Montecelio is approximately 40 miles respectively 64 kilometers.
By car or train, the actual journey to Guidonia Montecelio is certainly longer, as only the direct route (as the crow flies) between Terni and Guidonia Montecelio has been calculated here.
Air line (approximately)
64 kilometers
34 nautical miles