Strange decrease in mpg with VX when driving 65 mph instead of 72

[jadziasman]

Attention all VX owners. I noticed a very strange phenomenon with my mpg when traveling to Chicago last weekend. I purposely slowed down to 65 to 67 mph (in 5th gear with the cruise control engaged) to attempt to get higher mpg. Just the opposite happened - it dropped from 45 to 41. On the way back from Chicago to Detroit I got 48 mpg at 72 mph. The wind direction was from east to west on the return trip - no wind assistance obviously. I have also noticed a slight decrease in mpg in my 54 mile daily roundtrip from home to work when I drive at 65 mph in 5th gear instead of my usual 70 to 72 mph to keep up with traffic.

What the hell happened? I'm thinking that strong lean burn at 65 to 67 mph results in higher NOx in the exhaust, which prompts the ECM to increase EGR to take care of the NOx. Increasing EGR results in a loss of power due to inert gas being added to the intake air/fuel to lower the combustion temperature. The ECU might have been forced to enrich the mixture to compensate for the loss in power to maintain constant speed (I had the cruise control on the whole time). This could be taking place in repeated cycles of strong lean burn/EGR/richer air fuel/strong lean burn/etc.

While traveling at 72 mph I could have still been in lean burn mode but with a air fuel ratio of 16 to 18 instead of 20 to 22. If this is true, then there would not be a demand for EGR to "stabilize" the combustion temperature to take care of the NOx emissions. At 72 mph, I'm still only at 2300 rpm which should keep me in the 3 valve/cyl operation mode (the two exhaust valves are always operational).

Does anybody have a PLX air/fuel meter attached to their VX? If I could get one cheap, I'd install it just to find out what happened this weekend. Has anyone else experienced this? It is really weird. Does the supermid show this tendency at all?