Assuming the engine running.  :-)

Just to add to the general discussion -- the most fuel economic speed
for most internal combustion engines is where the torque curve peaks.
When you put an engine in a car, then you have to take air resistance
into account, which is more or less proportional to the square of the
car's speed.  So in a car, then engine will be most fuel efficient at
a lower speed than the max torque point.

The only good way to determine this is to actually measure fuel flow
(or air mass flow, which is proportional) at various speeds.