Those of you who either have some game design/development know-how, or have studied physics beyond sitting in on high-school physics classes and doodling on the desk will probably realize that speed and acceleration in GTA V doesn't mirror real life physics. It may seem contradictory, but the reason for this is precisely to achieve realism. The size of the map, other in-game physical effects and relativity all contribute to the fact that the game feels a bit more real if speeds are a tad slower in-game than what they would be in real life.
One particularly curious and industrious GTA V player has taken the effort to work out the equation used to convert in-game speeds to real speeds, using one of the bikes from the game the performance of which most closely resembles the performance of its real life counterpart. The Hakuchou, one of the superbikes in the game is clearly modeled after the real-life Suzuki Hayabusa, not only in appearance but in speed as well.
Comparing the two allowed Belluani to develop the equation into which any GTA V speed can be plugged to see what it would be in real life. Very roughly, there seems to be an 8 to 10 ration going on with speeds, but the equation can produce much more precise results.
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Belluani has also put together, based on the equation, a small data table showing a set of speeds and their real life equivalents as a basis of comparison that you can refer to. Even so, the equation is pretty simple, so if curiosity ever gets a hold of you, all you need to plug in some numbers and you'll see what kind of a speed you ran.
Have you ever investigated any aspect of GTA V interesting from a game design and comparative point of view?