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Bad Fuel Economy Following ECM Update

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This is long overdue, but better late than never I guess.

Car info:
2012 Chevy Cruze LT Automatic 1.4L
66,2xx miles
Location: TX
Regular Gas(87 oct)
No codes or warning lights

Engine Air filter changed- Jan 2015
Spark plugs gapped- This past week
Tires properly aired(bought new July 2015)

I got this car with about 30,000 miles(Sept 2013), always filled up with regular gas, performed all routine maintenance. Prior to May 2015, I would get 30-32 mpg on the highway(70-80mph) with my lead foot. End of May 2015, I took my car for a TSB regarding the negative battery cable and they performed the ECM update(Recall 14801). Following this update, my mileage took a noticeable hit. From low 30s, it dropped to high 20s(27-28 mpg on highway w/ lead foot). Until recently, I thought this was normal. After doing some reading on here, I realized the update should not have hurt my mileage and should have possibly helped it some. Driving conditions and my driving habits have basically remained the same(distance, traffic, no cruise cont).

I checked the spark plug gap this weekend and they were all around .024-.025 and I upped them to about .030. Too soon to tell if it's made a noticeable difference, but I'll get a better idea this weekend if that helps on my long distance trip.

Any thoughts on what might have caused this?
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I just got another notice that I haven't done the tune update yet. Not sure I want to with how many that have had these problems. What does the dealer say when one reports worse MPG after these tunes?
Go fly a kite?
 
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