What is the actual speed of nerve impulses in humans?
Short answer
A conduction velocity of 440 km/h is possible in thick, myelinated fibers. However, this number is probably more representative of the upper range of conduction velocities, rather than a conservative average.  

Background
First off, there are a heap of variables that affect neural conduction velocities (in myelinated fibers) in complex ways (Waxman, 1980), including but not limited to:


axon diameter;
myelin thickness;
internode distance;
temperature;
axonal milieu;
age of the subject.


Having said that, in humans myelinated, thin A-delta fibers the average conduction speed was established at 19 m/s (Gyberls et al, 1983), or 68 km/h. A range of pain-conducting fibers exist, with different diameters. As a result, they range in their conduction velocities from 0.5 m/s (2 km/h for thin C-type fibers) to 120 m/s (432 km/h for thick A-alpha type fibers).

Hence, the 440 km/h is certainly possible in thick myelinated fibers. Note I just highlighted pain-conducting fibers here as an example, and other classes of neurons may feature even faster conduction in their axons.     

References
- Gybels et al., J Neurophysiol (1983); 49(1): 111-22
- Waxman, Muscle &amp; Nerve (1980); 3(2): 141–50