I've got a 2000 bonneville SE that is using a lot of gas. I use to be able to fill the tank and go 57 miles before the needle came off of "F," now I'm only going about 38 miles before the needle is moving. I replaced both O2 sensors and that didn't help. Anyone have a thought?

When was your last full tune up done (plugs, wires, air filter, fuel filter)? And, how about your oxygen sensor? Has it ever been replaced?

How many miles per gallon are you getting? The method you described isn't the most scientific.

It could be the Maf sensor or fouled plugs.
You can change the plugs and check the wires. You need a scanner to check the maf.
Also whats your engine temp like once warmed up?

The FIRST thing I do when my fuel economy drops is replace the spark plugs.

Replaced the spark plugs and wires today. The old ones had a lot of buildup on them and two of the wires broke off almost as soon as I touched them. I live in Erie, PA and was told the gas stations recieve a different gas mixture in the winter that create lower MPG. Anyone heard of this?

This should only be in the neighborhood of around a 1 MPG difference.

You should see better mileage with the new plugs and wires.
The cold winter weather and the winter gas formula will also kill your mileage in stop and go driving.

I'm about to replace my plugs, wires and air filter. I searched the tech forum and could not find the specs for gapping the plugs. Is there a more extensive article/thread on simple replacement of these parts? I'd also like to replace my O2 sensor and fuel filter, but am checking dealership paperwork to see if that was taken care of as my warranty expired a year ago. Currently 50,000 miles, almost exactly. If there are specific threads for replacement of O2 sensor and fuel filter, please advise? Thanks!

When did these two measurements occur? IF one was in warmer weather where the motor oil and transmission oil were warmer and had less resistance that could explain higher fuel use when starting the car cold and driving. It takes many minutes of driving for the transmission fluid to warm up enough to increase the mileage. Also alcohol in fuel lowers the mileage, in my opinion and experience.

Just a few extra minutes of warming up the car at idle will kill your mileage.

My Bonne has only driven about 5 miles this winter and I have used 1/4 tank of fuel.