Your loss of fuel economy could be directly due to you using 'premium'. The write up above was good but missed an important thing; Premium fuel burns slower, meaning it is designed and requires a higher compression to ignite. Now if your running a slower burning fuel in a lower compression or unboosted engine, it is quite possible not all the fuel is igniting in the chamber and actually lowering efficiency and torque. The computer will compensate somewhat, but not completely...Extended use could literally gum-up and /or carbon up your valves, pistons and cylinder walls, which can eventually turn to varnish, which will cause issues down the road....Something to ponder anyway.
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