The calculated distance (air line) between Serdar and Gonbad-e Kāvūs is approximately 134 miles respectively 215 kilometers.
By car or train, the actual journey to Gonbad-e Kāvūs is certainly longer, as only the direct route (as the crow flies) between Serdar and Gonbad-e Kāvūs has been calculated here.
Air line (approximately)
215 kilometers
116 nautical miles