Again, with the MPG, need to fix this Options

[Alchemist_57]

Okay, i did a tune-up on my 95 GT 5-speed. That included a new fuel filter, oil filter & oil, air filter, spark plugs, plug wires, upstream O2 sensor, downstream O2 sensor.
Also, i'm guessing my ECU was reset because the battery was left out for about 2 weeks as i worked on different stuff. After correcting my mistake of crossing the plug wires, everything started up and ran fine.

On my commutes to and from school, i had noticed that the fuel gauge needle went down faster than it did before the tune-up. I checked about everything. No leaks at the fuel filter. And i usually run 92 octane gas cuz in the past, it got me better mileage.
I've been looking thru all the past threads about this and can't figure out why my mileage got worse.

I also need to know how many O2 sensors my particular car has. I kno i replced the two that you can get at from the engine bay. But i read somewhere there's one on the exhaust pipe. I've never had my CEL come on since i've owned my celi, so i dunno.

I keep my tires at 30 psi cold. I drive reasonably. Anyone got any ideas? I don't wanna pay for premium gas if it's going to give me the mileage that regular did.

Thanx,
Bobby