Mileage Uphill and Down

So, I was in Wash. State for a while this month and rented a Prius. I decided to check some milage on mountain roads, and here's what I got. All of these are indicated by the computer, so they are probably 5% or so high. Unfortunately, I did not see either the maximum incline or average incline of these roads posted anywhere, so unless someone else knows them, these numbers are not worth too much scientifically:

UPHILL:
Going uphill on I-90 to Snoqualmie Pass from the West (from Seattle), at 70 mph (the speed limit), fairly steady, with a few decreases for traffic and curves: 18.6 mpg.

Going uphill on US 97 to Blewitt Pass from the north at 50 mph, also the speed limit: 27 mpg.
Going uphill on the Olympic National Park road to Hurricane Ridge, speed limit varying between 35 and 25, with a couple of sections as low as 15 mph: 22.1 mpg. (Maximum incline here might have been 6 or 7 percent).

On the Hurricane Ridge road, I reset the Trip Meter A about a mile from the top (interestingly both segments of the uphill trip registered 22.1).

DOWNHILL, mostly:
The Hurricane Ridge downhill produced an interesting number, because I didn't reset the thing at the top, so I had a mile of 22.1 mpg at the beginning. You go down for about 18 miles, losing about 5200 feet of altitude to nearly sea level (Strait of Juan de Fuca). We kept driving, and at 24.7 miles (so about 6 miles of driving near sea level), the mileage gauge still showed 99.9 mpg. (I took a picture, but it's too fuzzy to put in here.) It was a couple of more miles before I noticed that the trip mpg was down to about 91.

 

Thanks, Bear.
I'm having a bit of trouble reading the numbers. Do you subtract two consecutive "Elev." figures in your chart to get the net elevation change between the two places?