I don't think they'd be limited by speed... the majority od dragons who warrant having 'range' would have subordinates who would be able to look after their affairs in places quite a distance from their lair. Ranges would probably be more defined by geologial stuff - black would control an entire swamp, buut won't care much about non-swpam stuff... unless, like in Dragonlance 5th age, they started to ex[pand the swamp. Likewise green dragons would control a large forest, blues a desert. The only exception really would be reds, they'd just want to control everything.
Overland movement divided by 2 is a good starting place, but those values could doublee or even triple depending on how many, and what type, of creatures call that dragon leige.