As far as I know, the key difference between iRobot Roomba 655 and 645 is that the 655 features the on-board scheduling whereas the 645 does NOT have the scheduling.
Extra Accessories
The iRobot 655 comes with 1 extra Side Brush, 1 extra Bristle Brush, and 1 extra Rubber Brush whereas the 645 does not include extra brushes.
Price
The 655 is $20 more expensive than the 645; the 645 is a Sam’s Club exclusive unit, while the 655 is sold at Costco.com.
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If you know other difference between iRobot 645 and 655, please share below.
I just bought a iRoobot 645 and want to know if the replacement parts for the 600 series is what I need to buy for it. Thank you.