Why isn't my 2005 Honda civic hybrid giving me good MPG?

I have a 2005 Honda civic hybrid notting is wrong with it my dad is the first owner he takes good care of it we replaced the battery that's in the back seat like a year ago this car is in good condition my dad used to drive it to work only it has 231K on it. My dad bought a new ford so he gave my brother the car. I already have a 97 Lexus lx450 which we all know is bad on gas 12city-13hwy. But anyways my brother pumped premium gas full tank from 76gas station because he taught it will give me better gas millage because this problem been like this for a month. It used to give my dad 38hwy and like 35city but now it gives my brother sometimes 24hwy,27hwy,29hwy-30hwy same thing as with the city. We don't know what's wrong can someone please help us.

First, if it doesn't require premium gas, you are actually hurting economy. Doing so indicates someone who doesn't understand octane or compression.

The key here is that someone else is driving the car. Easy solution is give it back to your Dad to drive for a tank, and see if that solves it. Most people can't drive anymore, and lower economy now is just a given, and I almost guarantee you that is what is going on here.