The calculated distance (air line) between Clausthal-Zellerfeld and Dortmund is approximately 125 miles respectively 201 kilometers.
By car or train, the actual journey to Dortmund is certainly longer, as only the direct route (as the crow flies) between Clausthal-Zellerfeld and Dortmund has been calculated here.
Air line (approximately)
201 kilometers
108 nautical miles