I am wondering what number the computer assumes for MPG in calculating how many miles are left from the current tank of gas. ?

 

based on your recently mpg and your driving habit. that's why if you drive a lot of freeways, you will see the range keep on climbing from tank to tank. the other hand with local, the range keeps on dropping

 

I believe it looks at your current rate of consumption and then multiplies that number by the amount of gas remaining in the tank. So it assumes that you will continue at that rate of consumption.

If I'm driving on the highway for a period of time and then get onto city streets with stop and go traffic, I'll see my cruising range drop as the computer accounts for the increased fuel consumption.