I recently had the opportunity to drive up out of the Mississippi River valley on a 4 mile steep grade. Remembering your comment, I paid attention to see if my Sonic running on 87 Top Tier fuel, would need to downshift while climbing the grade with the cruise set at 73 MPH.
You took my quote out of context, at 73MPH you are into the powerband enough in 6th gear that the car should not need to downshift on almost any grade. An example if this, I drove my Cruze from Wisconsin to Texas and back, on the interstate I never found one hill grade that required a downshift at the 78MPH I had the cruise control set at, my car never left 6th gear, ever on the highway in more than 4000 miles. SIDE NOTE: I drove that same Texas route with a GMC Terrain, its 2.4L 4 cylinder without a turbo at the same 78MPH, I needed to downshift on all interstate hills to maintain speed(probably more than 1000 times over the 4000 miles), makes sense though since that engines torque peak is 4900RPM. The turbos much lower RPM torque makes the Sonic and Cruze feel more refined than the equinox/terrain when drivign in hills.
Try that same hill at 55-62mph, driving some of these hills daily I notice this: If I leave the car in D the computer will not downshift with premium gas yet will every single time with regular gas it downshifts for the same grade as soon as I hit the hill.
This weekend I put 600 miles on In hilly SW Wisconsin, I seen 6-10% grade signs everywhere I drove. I ran regular 87 octane this weekend and just like every other time I have my MPG was much worse than my typical average driving in this area(2-3MPG less). My 2015 Sonic seemed to run the same most of the time however on hills if I was in 6th gear at closer to speed limits(55-60MPH) I would loose about 1MPH every 2 seconds of hill I attempted to climb when in 6th.
I should add the first day of hills it was 68F outside and I didn't notice much difference(I also believe the mix of 87 and 93 I had in my tank put me at mid-grade 89 octane level). The second day with 9 more gallon s of 87 octane also seemed fine until it got above 78F, the loss of power on hills seemed more pronounced.
opcorn: