I should have tested at 85 km./h, not 55 mph.
Why? Because all of my other tests have used metric speeds, and I can't compare today's numbers with past tests done at 85. (55 = 88.5 km/h).
I did it at 55 mph because I was barrelling towards the "start" line when I suddenly remembered I hadn't reset the
SG to report in metric. So I just set the cruise to the nearest multiple of 5 and kept going. Lesson learned.