michael--I find it hard to believe that even
highest available cruise for short periods would be ground level. the
air resistance is pretty high down there.

Nevertheless, it is so. The highest available speed is always at the
highest altitude where the engine can make full power. For an engine
that is air-limited (which is almost ever normally aspirated engine)
that is sea level.

For engines that are power limited - meaning turbocharged with a flat
rating - it's the maximum altitude where the maximum allowable manifold
pressure can just be attained.

Michael