Fuel Question

Okay i have a 90 lx 4dr and i have a fuel problem.

Okay when i got this car the lady that owned it always used premium fuel in the car and then about a year when the gas prices jumped she went down to using Regular. Okay when i got it i was just using regular and i was getting like 35 mpg. literally. On regular
Okay i want more power so im using premium now and i get like 20-25 mpg tops.
Whats the deal? Someone said i have to reset the ecu when you change the types of gas or what should i do?

The only mods i have to my car are headers, exhaust, cai. Any thoughts
Thanks!!!!

Ocatne levels really wont matter in a stock low compression engine. The reason your getting worse mpg is because of the I/H/E - you drive it like its a race car, I would guess.
Is the vehicle current on maintenance? Plugs, cap/rotos, fuel filter, etc. . .

Give it a tuneup and see what happens.

Plugs, cap and rotor, O2 sensor, fuel filter, etc

Thanks i will do that
And also when i mean i want power i dont mean that when i drive i start in first redline it shift into second redline it and so on. I dont do that i dont drive it like its a race car be heck it has a d15be dpfi. What do you think i would expect. I used to be able to drive it the same way as always and i still got great gas milage. So im going to give it a tune up. Thanks guys for all of your help.