Terrible Gas Mileage

[sooner]

Help!!! I have a 2003 Vibe with an automatic trans (also an 03 5 speed). It has over 110,000 miles. It is driven about 80% on the highway, around 70MPH. For over 4 years, it averaged in the high 20s (27-28mpg). In the past few months, the mileage has plummeted to 18-19 on the highway. Today, it registered 18.5 on 100% highway driving. Changed the plugs after it dropped but it didn't help. Starts fine, no driveabliity issues, no yellow or red lights on the dash, no smoke. I use top quality gas (76 and Chevron), Mobil 1, keep the filters clean, and the tires inflated. The transmission seems fine, with no jump in the RPMs from before (just under 3,000 RPMs at 70 MPH).My first thought is the O2 sensor but an idiot light should have appeared, right? Mr. Goodwrench wants to run tests ($105 for starters but no promises it will reveal anything) but was hoping someone has experienced a similar situation and could short-cut the diagnostics.My other Vibe is killing off headlights every 2-3 months (another post under electrical) but at $4.55 a gallon, this one is higher on the priority list. It's been a great car with the exception of this.Thanks in advance for your suggestions.

[zionzr2]

run a bottle of Techron concentrate.Also might be a plugged cat.

[Raven]

+1 on the Techron. You should also have the cats pressure tested but try the Techron first.

[joatmon]

viewtopic.php?f=17&t=6088&#p65128 has an excerpt from the 03 service manual on what to check if gas mileage is lower than expected.

[sooner]

Thanks for the excerpt. I'll check the coil and possible vacuum problems. It runs great (otherwise) and it is defintely not the gas quality, plugs, driving habits, wind, etc. Mileage is down from 28 to 18-19 with nothing changing from previous behavior.