You actually want to compare DISTANCE traveled over and under 40mph for your calculations.
If you grab the spreadsheet linked
here, you'll notice that for the LA92 + EPA FTP drive cycles that it uses, 7% of the distance traveled is over 60mph, 25% is over 50mph, and 40% is over 40mph. The data is from epa.gov.
My own commute? It's 2mi in an urbanized 35mph zone, 4mi in a PSL 45 where I can go 50mph, then 3 mi in an awful village with PSL 30. Then there's time I spend coasting down to red lights and stop signs. The ScanGauge says it's 22mph average, but I'll characterize it as 38mph distance-weighted average.