This is the strangest way to increase fuel mileage that I've personally experienced...
About 4 weeks ago, during a heavy downpour, I drove the car through a flooded road at a good speed; saw the water pond too late to slow down. Anyhoo, check engine light came on, and as I put it in diagnostic code, I saw MAF, Idle Air, and both the O2 sensors reporting errors. Car would have severe loss of power, and would stall out. I drove the car for another 20 or 30km before getting home. Car had essentially swallowed some water. Gulp.
I found the next morning that the air filter was still fairly wet, and was deformed from the suction of the engine. So off I went to the local parts store, and replaced the air filter.
This is where it gets interesting. Prior to the incident, I would get consistantly 240-260 miles before the fuel warning light comes on. After the water ingestion, I've gone thru 5 tanks of gas, and each I've been able to drive to 260-285 miles before the light comes on. That's a 10% improvement in mileage. WOOHOO! No driving habit change, nothing else has changed. Didn't even reset the computer by disconnecting the battery.
I asked my mechanic last week, and he told me that the water could have cleaned out some of the carbon buildup inside of the engine. He tells he occasionally he will inject water into the throttle body just prior to a car going for emissions test, to, um, steam cleain it.
Weird.
Remember, Your Mileage May Vary. LITERALLY.
VK
My 1.3 cents.