As I mentioned in another thread, I'm now using my "A" trip meter for tank mileage and my "B" meter for shorter segments. Today I reset my "B" to see how my mileage would be effected by strong Santa Ana winds. At the exact same time as I reset "B", I noticed that the "A" meter showed 50.5, with about 300 miles on the tank. After riding 25 miles into a headwind, I was pleased to see my "B" meter showing 52.2 mpg. Now the mystery...

When I went back to "A" to see how much the segment improved from the original 50.5, I was shocked to see it showing 50.4. How is that possible? If I added a segment which was above the average, how could it bring the average down? Seems totally impossible to me.

25 miles later in the ride, now getting a good push from the winds, my "B" meter was showing an impressive 58.0. The "A" meter had now come up to 51.2. I'd have to work the math on this one... I understand a 50 mile segment would only have one seventh the impact of a 350 mile one, so the 51.2 sounds more in line. I just don't understand the earlier situation.

Has anyone else experienced this type of mystery?

 

I've not seen this situation, but I'd bet it is simply a rounding issue.

Your ending situation (350 miles into the tank) should have driven the MPG up to 52.4. so.. chances are that your 50.5 may have been closer to 50.4. And maybe your 52.2 was close to 52.1 as well.
That's the danger of using small sample-size (in this case, miles or 1/2 gallons used).
Just my 1.51 cents worth.