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Having used regular unleaded for the entire time I've owned the car, I recently tried premium gas for about 4 straight tank fills. I noticed an improvement in performance, but it seemed like the entire time it had premium, the engine light was on. When I switched back, it went off. Maybe I just confused it. Why would this happen?
I have a 2012 LT1. Only owner, now has 115k miles. I've noticed a decrease in mpg ever since about 90k miles. I changed spark plugs after 100k. Didn't seem to help the mpg. I've always used regular unleaded since the user manual I think recommends it, but according to other threads, maybe I misread that.
With premium gas, I noticed an improvement in acceleration with the CVT (I use the manual a lot more as it's gotten older due to sluggish acceleration), and I got a 2-3 mpg increase. Up to about 90k miles I had averaged 31.5 mpg, now it averages around 29. With the premium gas it was back up to around 32 mpg.
 

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What's the code? Doubt it's fuel related; probably something else going on.

Auto parts stores will read Check Engine codes for free, but do not trust their diagnosis or throw parts at it.

Transmission isn't a CVT. It's a conventional automatic.
 
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