With areas like the Keweenaw and Ironwood being 9-10 hours away from the capitol, those 2017 speed limit hikes easily save 1-2 hours of drive time. If they were able to apply those hikes more universally, it would save even more.
I'm not seeing how it would save you 1-2 hours of drive time, maybe about 45 minutes but 1-2 hours seems to be stretching it.
Starting from downtown Lansing, a drive to Houghton is 495 miles. A drive to Ironwood is 540 miles. Out of simplicity's sake, let's ignore the relatively short sections of sub-55mph highway and assume you can average at least 55 mph the entire distance. I'll deduct the time savings of the speed limit increases from that figure.
Between 1974 and 1986, you are legally restricted to 55 mph. Drive time without any stops or other slowdowns, Houghton (not the lake) is a 9 hour drive from Lansing, and Ironwood takes 9 hours, 48 minutes. The portion from Lansing to the Mackinac Bridge is 230 miles and takes 4 hours, 10 minutes on its own.
By the end of the '80s, you could travel 65 mph north of Ithaca to the bridge. So for 188 of those ~500 miles, you can travel 65 mph. That alone saves a half-hour of drive time.
By the end of the '90s, the St. Johns bypass was completed and speed limits on the freeways were bumped to 70 mph. Now we have 215 miles one could travel at 70 mph. Travel time from Lansing to the Mackinac Bridge drops to 3 hours, 23 minutes from 4 hours, 10 minutes, for a savings of 47 minutes.
Fast forward to the end of 2018, after all the speed limit hikes take effect. Lansing to the Mackinac Bridge now takes 3 hours, 8 minutes, which is a whole hour saved right there. Haven't even taken the U.P. into account yet.
Taking the fastest route from the Bridge to Houghton (US-2, M-77, M-28, US-41), there are now approx. 150 miles of 65 mph speed limits. That saves you 25 minutes of drive time. Between Ishpeming and Chassell, another 75 miles could be reasonably posted for 65 mph, which would save another 12 minutes.
Lansing to Houghton, 1986: 9 hours
Lansing to Houghton, 2018:
7 hours, 33 minutes - 1 hour, 27 minutes saved
Lansing to Houghton, all available 65 mph sections: 7 hours, 21 minutes - 1 hour, 39 minutes saved
Looking at Ironwood, I'll favor the northern path along M-28 to maximize mileage at 65 mph. I can traverse about 228 miles of the 303 miles at 65 mph. Drive time to cross the U.P. falls from 5 hours, 30 minutes to 4 hours, 52 minutes, a savings of 38 minutes. Bump up the US-41 stretch into Ishpeming to 65 and you'll knock off another 6 minutes.
Lansing to Ironwood, 1986: 9 hours, 48 minutes
Lansing to Ironwood, 2018:
8 hours, 10 minutes - 1 hour, 38 minutes saved
Lansing to Ironwood, all available 65 mph sections: 8 hours, 4 minutes - 1 hour, 44 minutes saved
Note that in both cases, it takes half-again as long to cross the U.P. as it does to get from the Mackinac Bridge to Lansing. Not only is it shorter in distance (to Houghton, you travel 260 miles in the U.P. v.s. 230 miles in the L.P..), the 10 additional MPH you can legally travel on I-75 and US-127 does a lot to shrink drive time across the L.P.