Actually 30 - 35 mph is the point where the mpg starts to drop below the 99.9, and I am starting to consider that I am not getting much more than 100 mpg even at 70 since I did some more calculations:

15 MPG up the hill for 100 miles = 6.66 gallons of gas
100 MPG down the hill for 100 miles = 1 gallon of gas
200 miles at 7.66 gallons of gas = 26.1 MPG

I went up about 2 mile (15 MPG) and down about 2 miles (99.9) on a hill and got just about 26 mpg, so actually 99.9 might be close enouph.

My original calculation of:
(15 MPG * .5) + (100 MPG * .5)=57.5 was proven to be erroneous on a math board.

That calculation would work only if the amount of gallons was the same, since 6.66 and 1 is not the same it didn't calculate correctly.

True it probably is calculating the mileage, but only displaying the nn.n limit. After 5,000 miles it is very accurate, but at under 500 miles I have been over 2 MPG off from the gas pump receipt. I just wish I had that 100th place on the display.

I suppose if we could get the amount of gas consumed just by idling for an hour, we could actually calculate what mileage we might get at coasting. Although as you said coasting at different gears might get different results.