Poor fuel economy

I have a 2003 focus se. For the past few months my mpg have been around 25 the usual was 31. Every once in a while it improves for one tank. I have changed the trans fluid and filter, spark plugs and wires, fuel filter to no avail. Any suggestions

Only if your routes are exactly the same every tank, you cannot expect the same fuel economy. For instance, last tank I drove 30% highway miles and got 4mpg better than when I usually drive 20%. However, I also changed my driving style to allow for better economy.

What transmission do you have? 25 sounds pretty normal for a city driven automatic Focus. For a city driven manual, I'd say 27-29 sounds pretty normal for city driving, if you drive 'normally'.

Personally, I accelerate at about 1mph/sec. That is quite a crawl. Takes me about a full city block to get to 35mph. I also coast in neutral down hills, to stop lights, etc. Also turn my car off at lights, because they're very predictable around here.

Where are you located? I am assuming you live in the northern hemisphere has winter driving possibly affected you in some way? To be honest, I think 31 mpg sounds pretty hi for city/highway combined driving for a 03 focus.

You should note that the mpg numbers listed for a car are the average. so if a car is listed as 22/30 it really means city: 20-(22)-24 and hwy: 28-(30)-32. Also as a car ages it begins to become less fuel efficient.

Do you have a lot of junk in your car? I dont mean like paper or candy wrappers but heavy equipment, tools, sports stuff, suit cases, etc. If you are carrying an extra couple hundred pounds around regularly then that could be the reason for the drop in gas mileage. That would also include car pooling. with three extra people that would add about 450-500 lbs no problem. That would result in a noticeable fuel drop as well. Also things like using the heaters or AC cause most vehicles to lose a minimum or 1-3 mpg. And lastly: tires? have you been using chains or really large off road wheels?

While a drop of 6+ mpg wouldnt probably occur from one of these individually, one or more of these put together could easily explain it.

Thank you the replies. I drive the exact same route everyday about 80 percent highway using cruise control at 65 mph. the car is empty, no junk. I live in northern Virginia, commuting about 65 miles one way. I keep a mpg log in my car to track my fuel economy with date, miles driven, and gallons at fill up. I always fill the tank completely. The tires are less than one year old and i check air pressure with oil changes at every 3,000 miles.

Well then, here's what I would suggest:
Driving more economically
Using a steady foot instead of cruise control

Some things that I do:
Downshifting when coming to a stop, instead of using brakes. I only use brakes once I'm below 10mph, usually slower.
After downshifting thru the high RPM's, I turn the car off (usually in 3rd gear), and continue downshifting with the car off. Sometimes I can avoid downshifting when the car is on by putting the car in neutral and turning the car off far enough ahead of the stop. This allows me to slow down before downshifting, without using gas or braking power.
I turn the car off at every red light. (I cannot advise you to do this. This is what *I* do.)
Accelerate at 1mph per second.
Coasting down hills in neutral with car off.
Here's my gear shifting on flat roads (mph may not be exact) — 1st 0-5mph, 2nd 5-13mph, 3rd 13-19mph, 4th 19-28mph, 5th 28mph+.

Using these techniques, I have been able to get 32mpg with 80%+ city driving for the past four tanks.

If you think you're doing all you can on your end, then you need to see a mechanic. I am no mechanic. I do know that a fuel leak could be the issue. Or a fuel pressure regulator. Possibly even something in the ECU.

Sounds like they have changed over to the summer blend fuel. If that is the case it is watered down with Ethanol and it helps to reduce your mpg. No amount of grandpa acceleration and all those other methods mentioned will help. Yes it sucks but it is what it is.