when driving my 2006 ford explorer today i watch my gas mileage go from 100-68 in lest that 2 miles of driving. what could cause that?

The on board computer system which calculates the "miles to empty" takes many readings into consideration. First, and most important, it assumes the vehicle is operating at peek performance. It also takes into consideration "driving cycles" as it relates to MPG's obtained. As an example, on a cold winter morning if you start the car and let it idle to warm up the heater, your effective mpg's during that time is zero. Likewise, a short hop to the local convenience store is going to lessen your mpg's, especially if a stop light or two is involved. The calculations are based on driving cycles. It is also good to remember the manufacturer errors on the side of caution. Meaning if the system displays you have 29 miles to empty, you may have two gallons or so remaining in the tank.

The fuel gauge sensor may need to be replaced. There were a few service bulletins for 04-07 explorers on the sensor not working properly.