Notes:266 knots ground speed sustained for 13 minutes or so with excursions up to the low 280's.
I was on the way to a Beechcraft owners' function (BPPP) in Lakeland, FL from Lafayette, LA, at 15,000; straight, level and smooth.
These best ground speeds were from a straight downwind leg on a day after a cold front came raging through the Deep South.
The airplane is a stock, unmodified 1979 B55 Baron with IO-470L's. Photos of the steam gauges were taken about a half hour earlier on a more easterly heading and show the engine settings and indicated airspeed for the entire flight.