Bad Gas Mileage

I get 18 1/2 mpg in town and about 25 mpg highway and city combined. I`ve tried intown driving in Overdrive and in regular drive (3 on selector). I get basically same mileage in town. I have used Lucas gas treatment once and am getting ready to change my spark plugs. Has anyone had similiar gas mileage and had it remedied somehow? Air filter is like new and I get regular oil changes. Would an engine diagnosis scope help to determine the problem maybe? I don`t drive it hard at all either.
Just getting some ideas to get closer to the 21 mpg in town and 32 highway mpg that the car is rated for. I drive quite a few miles per month.

Thanks!

You didn't say which engine you have, but it doesn't make a large difference. The biggest factor in gas mileage is how heavy your right foot is. Slow starts, coasting to stops, easy acceleration, combining errands into one trip...all help.
After that, I'd check tire pressures every week or two. Emptying the trunk/back seat of unneeded weight will help also.
You should just leave it in Overdirve all the time. By driving in 3, you are preventing it from shifting to the higher, more efficient gear when conditions allow.
I get 23-25 combined town/highway mileage in my 3.8 2000 Impala. From what I've read, your mileage sounds pretty normal.
The numbers on the window sticker are obtained by driving a specific test cycle, and don't even come close to real world driving conditions. The test drive is never over 55mph, no air conditioner use, no stop and go traffic, no stuff in the trunk etc...those numbers mean very little, and you will probably never bea ble to match them in the real world, sorry.

I have the 3.4L. It seems like it doesn`t have the power it should. I drove a 3.8 LS model a couple of weeks ago and it was the difference between day and night. Maybe the 3.4 is not a great motor for 2004 Impalas.

Same here dh271. Up until last Sept. Oct. I was seeing up to 34mpg on highway and around 22-25mpg in town. I have been watching the forums and have not found anyone with a sure fire way to return their gas mileage to "normal".

Here is what happened with my car (to compare to what is going on with your car). In one fill up I went all the way down to appx 22mpg on the highway. At the time I filled up the MIL came on but turned itself off. I figured it was the gas cap not being on correctly and thats why it went off. I have scanned the system with an OBDII scanner and it found no errors. The first scan I did everything checked out but on a subsequent scan it told me that the evap system could not be scanned. Until I have the money to get it to a shop and have it properly diagnosed I guess I will just have to fill up more often :( As if the price of gas wasnt bad enough!