Sphinx canadensis, New Brunswick, Canada, courtesy of Reginald Webster
Sphinx chersis ??, Athol, Worcester County, Masachusetts, July 19, 2010, courtesy of Dave Small.
2) Near the forewing anal angle, the black streaks seem considerably wider (more pronounced) in canadensis than in chersis. Also in canadensis (more visible on left forewing) the same black line continues in two teeth to the inner margin, whereas in chersis the same line simply seems to fade into oblivion.
3) The black longitudinal band on the abdomen in canadensis seems continuous from the tip of the abdomen to the thorax, whereas the same band is only present on the lower abdminal segments in chersis.
4) The hindwing fringe in canadensis seems predominantly dark grey with slight off-white checkering, whereas in chersis the hw fringe is predominantly off-white with darker scaling only along the wing veins.
Dave also sends the image below, featuring the moth in question next to a Ceratomia undulosa. The large size is suggestive of S. chersis. I think S. canadensis is much closer in size to C. undulosa.
Sphinx chersis ?? and Ceratomia undulosa, Athol, Worcester County, Masachusetts,
July 19, 2010, courtesy of Dave Small.