As soon as “serious” conclusions (with respect to reality) have to be drawn from virtual reality experiences (training, virtual prototyping…), it is now more and more acknowledged that, besides display calibration and computer graphics issues, some attention has to be given to perceptual calibration, on the human side. This paper presents results from recent experiments that extend previous data on speed perception during driving simulation. They show 1) that the manipulation of the position of the rendering (virtual) camera strongly influences the drivers' speed perception, by transforming the optical flow pattern and 2) that this manipulation remains unnoticed by the driver and does not impact his/her attitude toward the simulation. They suggest that the position of the driver's viewpoint, with respect to the simulation screen, is of critical importance for the calibration of ecologically valid simulation systems. More generally they emphasize the fact that perceptual calibration is fundamental in “serious” virtual reality applications.