I understood also. The computer dos not use only the speed; it uses RPM, speed, horsepower, fuel trim and probably more; your foot uses what your eyes see.
One thing I found that limits it is don't set the speed when accelerating or going uphill.

 

I'm with FMG. Get a 5-speed SS, it'll go right up in 5th gear. Maybe the automatics do also (without downshifting, I mean).

But seriously, your 2.2 doesn't make a lot of power. When it drops a few miles per hour, the cruise control says, "oh crap, really gotta get going here!".and so gives it more throttle than you would manually (because you monitor your speed, and don't let it drop that much), and the downshift, and stays in that lower gear until it knows it can maintain speed with less throttle. And when it lets off the throttle, then it can upshift.

 

"I approach the SAME hill at the same speed without cruise, and the transmission doesn't downshift at all,"
The cruise module is commanding much more throttle than you are doing with hour foot. Step into it.

 

Yes, that is what I was trying to say Greg .

It seems to me we had someone here nearly blow up their engine on a long steep grade by letting cruise control take over.

 

I totally agree with you desertrat. Mine does the exact same thing. It is the only annoying thing about my HHR. When I got the Trivecta tune (years ago) I seem to remember he did the tranny tune too, and tightened it up. Maybe he should have loosened up the cruise control down shifts. I was going the get the stock tranny program from the chevy dealer, but never got around to it.

 

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