My average Gas mileage has drastically changed. Current Miles 5000, since purchasing my 2007 STS V6 my average mileage (Rural Driving) was 18-20MPG, consistantly, suddenly it started dropping consistantly within 1 month down to 15.3, and under no conditons did it ever increase.
Dealers response: No codes.

Since then 3 weeks have passed, under no conditions (long 60 Miles or more), no city driving, the Average Miles Per Gallon Never increases, and has Dropped to 15.1?????????

I don't think the transmission is shifting into Overdrive, but dealer says no codes!

Any ideas, seems dealer has no answers, but somthing is drastically wrong here. :bigroll:

Reset your avg. milage readout. What is your inst milage readout telling you? Are you checking the milage by mles Vs. gallons at the pump on refill?

Have reset the average MPG reading mileage will creep up to 15.1, never higher, as for instant mileage reading it does fluctuate, from low to high, which I assume works like a vacuum guage, high vacuum = high mileage reading, low vacumm = low mileage. Fuel consumtion via fill up method confirms low MPG!

My 2005 STS V6 always produced 18-20 Rural MPG Average, 23-28 MPG Highway travel

Where are you buying your gas....quality of fuel can change your MPG's.

Use same Major Gas Companies through out area that I have always used. This is definately Not a Quality of gas issue. Again I would point out that until last month, I always maintained a 18-20 MPG average on rural driving, and 23-28 MPG on expressway trips

The avg. milage readout is a rough approximation at best. The only way to be sure of your milage is to figure milles driven against gas at the pump at refill.

Our STS4 had dropped in mileage from about 20 MPG to 16 MPG on in mostly highway driving. It was also running poorly and the check engine light eventually came on. It turns out that the catalytic converter had deteriorated in under 2 years at around 60000 miles. It it being replaced today under a special warranty for emission controls. The dealer said he had seen quite a number of catalytic converter failures on the STS, so maybe that could be your issue.

With regards to gas, in some areas, gas companies switch from a "winter blend" to a "summer blend" which can have different levels of oxygenators and ethanol in them. I have had mileage decreases/increases based on the blend being sold in my area, but nothing ever that drastic.

That is very low MPG for your engine in any event. I have an STS-V and a V8 STS4, and both get much better mileage than you do in rural driving.

I would agree that if it is definitely not a gas quality issue, then something is wrong with your car.

Problem Solved, Dealer decided to give the car a thorough exam, and resulted in locating a damaged connector which controlled the shifting of the transmission. Overdrive was not consistantly engaging, he stated one of the Pin connectors apparently was broken or loose in its housing, and was replaced. Did not take long to see the results, on the rural drive home, jumped to 19.3 AMPG.............