I said that wrong. To get 3V you use two D cells in series and the mAH does not add. So it would be 12000, not 24000 for two D cells in series. The motor is rated a 3-12V so it will not run on 1.5V.That reise that question, if i use:
1 x D cell (1.5v) with 12000mAh,
2 x D cells that will provide 24000mAh and step it down to 1.5v output,
will the 2 x D cells setup provide double the usage time than the 1 D cell ?
You said in the original post that the manufacturer gave you a current of 0.17A at "maximum efficiencly". Maximum efficiency would be at 3V with no load. Assuming it light load, it might not be much higher than that, which is confirmed by the fact that it has run 3 weeks on two AA cells -- and not that I calculated 22 days on two AA cells at 0.17A.