Here’s another way to look at it, assuming they knew the original fuel load:
They could calculate the fuel usage given the know route and altitude for the time until it left radar range. Deducting that from the original fuel would give the fuel remaining.
Now, they know the approximate time the plane was in the air with that remaining fuel load. From that, they should have been able to deduce the plane’s average airspeed over the remaining time, knowing the fuel usage statistics of the engines involved.
For example, if you’ve got 20 gallons of gas in your car and run out of gas in five hours, you then used 4 gallons per hour. If driving at 60 gives you 15 mpg, and at 72 gives you 12 mpg, you know you averaged 60 mph because you were using 4 gallons per hour over five hours. If you’d have averaged 72 mph, you’d have used 6 gallons per hour and run out of gas in 3 hours and 20 minutes, but the 5 hours is a given so you couldn’t have gone 72 mph.
So knowing the fuel remaining and the time in the air would allow calculation of an average airspeed, just as driving 5 hours on 20 gallons would imply an average speed of 60 mph given the characteristics of the car engine involved.
For what it’s worth, I still this this is a crap shoot because if the pilot was still in control he could have varied the altitude which would blow the calculation. It would be similar to being on a 5 hour downhill grade in the car example. You’d get a lot better mileage.
I think this new search area is indicative that the pilot was at the helm at the crash and that it may have been a controlled crash in an area where the plane would not be found but also where the pilot could swim away.
If they find that plane largely intact that will be a big tell —