In reality, the difference can be much greater than 15% based on the quality of fuel. My Honda (which isn't even designed for premium) will do ~25mpg in city driving, but struggles to even do 20mpg on regular. On the highway it's even bigger of a difference - somewhere around 36mpg on premium vs 28-29mpg on regular.
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Something else is wrong with your Honda.
__________________ CENSORED due to not family friendly words