When you say Google, I presume you mean Map Quest right?
I did it both ways from St. Ignace to Duluth, shortest time/shortest mileage.

Not much difference, they both take you up to 28 from St. Ignace, then West on 28 to Wisconsin and up.

I think there's better route that avoids going thru the major cities along Superior and would be my choice.
IF you can get a physical road map of the UP, I think my alternative has some merit.
Might be a few miles farther, but you save the time not going thru the cities.

From St. Ignace, I'd take 2 WEST, to Rapid River, jog south on 2 and pick up 35N in Gladstone.
That takes you up to 28, west of Marquette.

I think you can probably average 60-65 on 2, the same on 35N and on 28, I'd just go with the flow, not the fastest/slowest.
Check it out and see what you think?