Speed vs time taken to traverse is massively skewed long. According to the readout on the first ATV I am ruinning about 23. There are no units, so I would speculate km/h, mi/h, m/s, or ft/s.
To give a gauge, I was travelling to a location 2000m (2km) away. At 23 km/h, this should take approx 00:05:13 in game time; at 23 mi/h, 00:03:15; at 23 m/s, 00:01:27; and at 23 ft/s, 00:04:45. In reality, this takes approx 03:00:00. In order to match this ridiculously slow traversal speed using any known units of speed, I would have to look at 23 as either cm/s (giving approx 02:24:56 to travel 2km) or ft/hr (giving 04:45:17 to travel 2km), which are not customary units anywhere that I am aware of.