THE simplest and cheapest device for improving gas mileage in any vehicle is a virtual egg!
Put one on your gas pedal and one on your brake pedal.
I frequently drive a van equipped with a "mpg" readout and on an 75 mile round trip to the Austin airport IN FREEWAY TRAFFIC I invariably boost the mpg readout at least .1 mpg, sometimes three or more tenths.
The readout typically runs about 22.5 mpg from other drivers, and this is after several hundred miles. Once, just for fun, I reset the readout when I started on a Friday afternoon--heavy traffic all the way to the airport and back. That Chrysler Town and Country managed 28.2 mpg.
The next time I drove it it was back down to about 23.
Just think of the incredible, squishable egg.
