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Rob215,
First, his is a much older XD (I just tried to call him and I am very sure the car is <2010.)
Note that he did not get uber mileage until he had 10-20k miles on the engine. Then, it got the big numbers.
Then is the possibility that he got a car that just so happened to have a special test engine. While never admitted to, some cars get engines that radically better gas mileage than the published specs. In particular the GM 3.8 - so of these made it to the road with lean burn tables already in their ECU and in some cases enabled.
I told him, he should keep his XD until the wheels literally fall off because I would bet money that he will get significantly poorer gas mileage with a new replacement that claims better. Remember, his car is EPA spec'd hwy at something like 34 mpg and people usually get less!
On level road, I was clearly getting 50 mpg but it was physically impossible to control the throttle position since it required maybe 1/16" at the pedal. The cruise would drift and pump resulting in less mpg.
One theory could be fly by wire engines. Is the 2011 HHR FBW? It could be that a FBW system would not attempt to maintain a "1/16" " throttle position since this is of the order of the clearance stackup between the pedal and the throttle plates. If FBW starts at "1/8" " you will never see 50 mpg. Its also possible that running and engine with just a crack in the throttle plates results in increased emissions for some reason maybe related to boundary layer flow. So to keep the EPA happy, they program that out. But that is wild speculation on my part. Who knows, maybe he got a special engine with lean burn tables enabled?
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