Whassup with gas mileage? The last four months, my mileage has sucked.

I used to get 350 miles on a tank (Honda Accord EX-6 cy, 2001 sedan). Now I can't get 300.
I took it to the dealer, they said they found nothing wrong on the diagnostics. I ran a search and found that lots of other people are having similar drops in mileage. I drive 100 miles a day. I can't afford to have my mileage drop this badly. I'm thinking of adding acetone, 2-3 oz a tank, to see if it helps.
The only thing I did different was buy new tires - but they aren't that different from the old tires.
Ideas???
Will acetone help?

I think the Bush administration is behind this. LOL

helps keep the emissions down they think, and robs you of power and gas mileage. Fall comes, methanol gone.

and it was ethanol, not methanol.

And they can put in up to 10% ethanol without telling you. The problem is, ethanol has much lower energy density than gasoline (like 50% less energy for the same volume), so 10% ethanol will instantly drop your gas mileage 5% even if absolutely nothing else changes.

The dealer probably just ran a diagnotic test and found no codes. Your car is at an age where a basic tune-up might be necessary. You want to rule out the cheap things first:
Air filter
Spark plugs
Spark plug wires
Timing (although this shouldn't be a problem with today's direct injection engines)
Tire pressure
Wheel alignment

Other things to ask:
Are you doing more city driving? V6 motors get way worse city mileage than highway.
Are you using the AC more? I get 450 miles w/0 ac on my accord and 390 miles w/AC.
Is your transmission slipping? This will cause a severe loss of gas mileage
Is the car overheating?
How do your brakes feel? Dragging brakes can cause bad gas mileage
Have you changed brands or octane of the gas you buy? Higher octane = lower mileage. Accords do not need 91.
Does your engine sound the same as before?

Putting acetone in the car will not help. Its like giving cough medicine to a sick child. It might mask the problem but will not solve the cause.

Spark plugs are great - perfect.
Fuel injectors are clean.
Air filter is new
Timing belt is fine
Wheels are in alignment and tires are fairly new, with 29.5 psi in each tire.
I drive 100 miles a day on a freeway, with very little city driving. I have the AC on, because I'm an asthmatic. The transmission was just serviced and it was found to be perfect. It isn't slipping. My car is not overheating.
My brakes? They are fine, but the brake shoes are always getting warped. I don't understand why. I've noticed a little bounce again in my brakes this week, meaning it will be time again to turn or replace rotors soon.
I have been buying gasoline at one of three gas statons - I was buying at Wal-Mart until someone told me it had ethanol, so I started buying at another location.
I honestly did see a drop from Wal-Mart gas, but just a little.
The V6 engine uses more gas. If I had a 4cyl it would be a lot better. But 22.5 mpg (my last tank) is just terrible gas mileage.
I should be getting at least 26-27.
And I've been slowing down and going the speed limit to see if that makes a difference, and guess what? It didn't.
I did see a noticeable difference when I used acetone, a bounce from 20 mpg to 24. But that was only one tank.
Now it has settled down to 22.5 or so.

I have one last suspicion. Your car sounds like it has a lot of miles. Your mechanic should do a compression test and a leakdown test to determine the condition of your pistons and cylinders. Many times, valves can become worn out and leak compression, which gives you less power. Your car then has to work harder to maintain the same speed.

It might be simply bad gas, but V6 Honda Accords should be getting more than 22 hwy mpg.

I'll have my mechanic do that. But, here's a query - say he finds something wrong with the compression. What are we talking? A new engine? What will this cost me?

If it is the pistons or rings then it's going to be a lot ($2,500-$3,500) or so. If it is the valves, then not too bad. ($500)

So it looks like I'll be buying a new car first. Thanks for the reply, though.

On edit, corrected spelling.

I had a Nissan something or other about 15 years ago. It was getting on in years (maybe 8 to 10 years old?). Cool-ass car, digital dash, fogs, etc. Anyway, mileage started dropping, to where I was getting barely a third of what I was originally getting.

It turns out that my gas line had pinhole leaks and was subtly spraying out gas. Not that you'd notice when you parked it, and not a single stream when you drove it (which might catch on fire) but sort of a constant driving spray that nonetheless had me losing gas as I drove.

Yours is relatively new, that might not be an issue, but if you're having it checked out, have them look for microleaks or one or two spots that would be just enough to bring your mileage down 15% or whatever it is. Just a thought, one more possible cause.