Once again bringing up Japan - reading meteorologist reports from WWII frequently forecasting 160 - 180 knot winds at 35,000 feet over Japan and having flights measure 240 knot wind speeds. Also the report of a B-29 weather plane making a routine mission beginning in the Marianas and ending up in the Aleutians due to determining the only possible airfield at which they could land safely being up there. Now adding 240 knots to 490 knot cruise has got to be an impressive ground speed - like 840 mph.