HELP: Gas Mileage Problem

Accord V6 EX 2002
43K miles
I'm new at this, and don't know that much about the mechanics of my car.
Before i took it to the local pepboys for an oil and airfilter change, i was getting about 350+ miles/tank average. Right after pepboys, I filled up my tank and reset the miles. After the first new tank, it barely made 300 miles before the light comes on. My driving routine is pretty much the same, no significant changes and i hardly use the AC. I went back to pepboys and talked to them, the jerk said that what they did couldn't have affected my gas mileage, not even changing the airfilter. Is there anything i can do on my own to find out what the problem is or to solve this problem, before i have to bring it to the shop? I looked at the airfilter today, it was upsidedown, the "UP" part was facing downwards. So i flipped that around. I'm not sure if that will harm the engine or not, since it's been 3 weeks from the installed date, but it looks clean. I don't know if that was the problem, I doubted though. I'm on the fourth new tank since I took my car to pepboys, and it's half way and only 103 miles. So at this rate, i'll be getting about 200/tank. I have heard stories like this with a bad fuel sensor, which is expensive to replace. I sure hope not. Could the fuel sensor be damaged easily? Anyway, any help would be great. I'm still bitter about Pepboys, and I can't even do anything about it, they'd brush me off, and i think i lost the receipt by now.

If you want to calculate your MPG, you need to divide mileage by amount of fuel you put in. fuel gauge isn't accurate enough to use it to determine MPG.
gauge on my car can 100ml right before half tank then I can go another 100ml and gauge will show a just below half tank. Gauge just not accurate enough, its there so you wouldn't run out of gas.
Incorrectly installed air filter and heavy oil can contribute to poor MPG. Do you know what oil did they put?

it appears on paper that they used the correct oil grade 5/20 as recommended. So that just leaves an upsidedown airfilter. Well, i'm gonna have to fill up the tank to see any changes in the mileage now. And yea, the gas gauge is not that precise, but should be enough to tell if it has such a significant drop in MPG like i'm experiencing about 100+/tank. I know my car holds 17 gallons, and light comes on when 15 is gone, leaving 2 gallons reserved. So i'm getting roughly 220miles before the light comes on with the last tank, that comes to about 15 MPG. That's bad, cuz my car isn't an SUV.

I have a 2001 accord ex v6, and I have had it for a year and half. The entire time I have had my car I have have been experiencing the same problem. At first I thought that it was normal for a v6, but after talking to other people I realized that I should be getting more miles per gallon. If you find out something please let me know.

Ok, now i'm near on empty on the new tank after i switched the airfilter. I'm at around 303 miles and the light hasn't come on yet, but from the look of the the gauge, i'm pretty close to it. Although I'm still not satisfied, but i'm a bit happy that it has gone back to what it was before.. so the freaking idiots at Pepboys F-ed up on me, should have done it myself. I hope i get about 350miles before the light goes on, but i'm guessing it will be around 320miles, that's by 15 gallons, so about 21 miles/gallon, and two reserved, which will kick in after the light goes on. Man, i wish i knew a cheap way to boost my gas mileage. I think i'll have to bring it into a shop for a major tune up, but maybe later, maybe around 50k/60k miles or something. Also, my brakes have been squeaking for years now, even after i had it changed, at a small local shop. And i think my car was brought into a honda dealer service by a family member during my school year to fix that squeak.. spent almost $800 including some misc. services. But the squeak came back, they say cuz the brake discs get hot, that's why they squeak. So f-ck it. As long as the brakes work. Anyway, I'm really disappointed with my car. I think i got a lemon. O well, i've always had the worst luck anyway. If any of you guys know any low cost ways to improve this accord v6, please let me know.

i do all maintanance and repairs by myself for one simple reason; all shops in business of making money. and if you only care about money you don't care about car. and that why you very often see loose or over tighten lug nuts, air filters upside down, loose wiring connections and etc.

BTW, your brakes squeak, because shop didn't put shims and anti-squeak compound.

I guess it really pays to familiarize yourself with the car. When you know the basic systems and characteristics of your car, it's much easier to diagnose and even correct shoddy maintenance issues. Speaking of those quick lube joints...don't use them! I've spoken to ex-employees in these forums. Shady business practices, cut corners and general incompetence seem to be the norm.

Listen to HONDA_99 advice, you need to divide mileage by amount of fuel you put in. That's the way i do mines.

Also, i hear many complaints about Pep Boys car service. Also, Jiffy Lube service too.