Highway driving, better MPG with or without cruise control?

Downhill doesn't count.

 

This varies among cars. My two most recent manual transmission cars had their peak MPG somewhere under 40 mph, and dropped quite noticeably at 50-55 mph. The gearing, particularly on the last one, is for performance and needlessly hampers MPG at highway speed. But both still did better than automatics of the same vintage.

The EPA chart of typical mpg vs. speed is probably strongly weighted to automatic transmissions, which have different loss mechanisms and different gearing. On the latest models, I've noticed a widening gap in gear ratios between sticks and slushboxes, with the later more strongly shifted to be efficient at highway speed.

That graph is consistent with three other sources, so I tend to believe it. The equation produced by the Japanese hypermilers puts peak MPG at about 15 mph.

Do you have other parasitic loads or climate control running? These increase the parasitic loss, pushing peak MPG to higher speed by more strongly hampering low speeds than high speeds. The other sources kept overhead losses to an absolute minimum.

 

Yup, with outside temp at -15C or lower (like today!) the A/C is on auto! And will stay on auto! Though I do shut it off at lights if the engine tries to run. Once the engine warms up it has little effect, but for shorter runs (less than 20 min or so) the engine takes some time to warm up so runs a lot. It also cools off fast when it stops. Grill blocking does help quite a bit, BUT "not enough".
Second effect is even though I'm in the "great white north" it's dark a lot in winter. So the lights are on.
Finally, most of my trips are shortish, less than 30 min of driving. So the above has a big effect.