My gas mileage went straight to $hit

Ok, 86 TC stone stock..other then a boost controller to 15psi. I drive 67 miles round trip to work each day. I used to be able to fill up on wed and drive til the next wed before I needed fuel. All the trips were to and from work nowhere else. Now, I'm seeing barely 280-300 miles on a 17-18 gal fill up. $20 here and there with gas at 2.89/gal is adding up. I realize it's now cold and fuel tables are in fuel enrichment modes but Im not getting crap for mileage. I have changed nothing since the summer. Surely the 17's I put on the car didnt affect it that much.

Less thsn 17 MPG looks really low. How long since you had a tune up? How old is your O2 sensor?

17-18 is all I ever get with my '88 automatic. Gas up here is in the $3.50 range for super. I do outside sales and go through a tank a day most days. The '88 5-speed I recently got is a little better, but has 373's out back so could do better on the highway. I see guys getting 25-30. Not here. Both cars have fresh tune-ups also.

Also remember that winter blend gasoline does not get the mileage that summer blend does. Tuneup plays an important part. Tire pressure can make a difference also. Also pull the codes. How dirty is the air filter?

Try a grounding the O2 sensor with a wire to good ground, the mani is not always a good conductor. Got my mileage from the low 20's to almost 30 highway.

tune up is 2 months old, and all Motorcraft parts. Plugs/wires/cap/rotor/filter/PCV valve. 02 I'm assuming is orig..its the single wire version. Tire pressure is kept at 32psi..I'm pretty meticulous about that. I run 87 in it just keep my foot out. I will pull codes at work Monday, air filter is new. I was doing great in the summer, just seems I'm filling up ALL the time. The rear end needs rebuilt and I'll be using the 88 3.73 disc rear in it, spins 3500 at 75-80 now..so I'm afraid what the 3.73s will change it to. I'm putting the low fuel light to the test and driving for as long as I can with the light on. I wait til I round a corner and the car sputters..then I fill up hah!

I averaged 22 mpg on the tripminder, most of the driving was to/from work, not quite 30 miles, and mostly highway driving. After a new O2 sensor, and a full tuneup, Im showing 28 mpg average on the tripminder. Im even running through the gears more now and still getting good mileage.

The rear end needs rebuilt and I'll be using the 88 3.73 disc rear in it, spins 3500 at 75-80 now..so I'm afraid what the 3.73s will change it to.

The EEC richens the mixture above 3K rpm

3.73's with the stock rim and tire sizes on my 88 nets me 75mph right at 3k. I cruise on the ragged edge of open loop at 3k. Above 3k my mileage goes down at least 4mpg; partly because I'm running higher than stock fp and the rest because (like Pete says) this is where the O2 is not used by the EEC to meter fuel any more. Pre-programmed fuel maps are used above 3k which results in "more rich" than the switching o2.

I run my 88TC stock size rims/tires at 35 PSI, 3:55 rear gears on the 5 speed, I use oxygenated premium gasoline, (Usually Chevron for the Techron) full or semi synthetic oil, stock air cleaner/filter, injectors, timing, FPR, fuel pump and all Motorcraft tuneup parts. My commute is 80% highway 20% city streets and I average 24.5 to 25 MPGs on each tank of fuel. I check my MPGs with a calculator on each visit to the gas station. On long highway trips using the cruise I can get 30 MPGs on average. I move at the pace of traffic and I don't take any extraordinary measures to improve gas mileage. My TC has been very consistent on its MPG. If you had a recent drop in fuel economy something must have changed. Do you consistently check your fuel mileage?