Is this speed correct on a 777?

611MPH is achievable with a 50mph tail wind.
They get much higher than that at FL340.

I’m sorry. I had it on the wrong spot on the graph. If you look further back it was showing 1000 mph. A sharp increase in speed then a steady decrease.

I’ve guessing that is an anomaly due to a blackout period caused by the location. In other words, it looks like noise to me. The steady decrease I don’t think is a steady decrease but simply connecting the dots across an area of no data.

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That’s exactly what happen. Tail winds are usually going the other way around.