The formula with 3.1 substituted
miles = 3.1 ÷ 1.609344 = 1.926
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Kilometres to Miles Converter explains where each comes from and gives a longer table.
At a walking pace of 5 km/h, 3.1 km takes about 37 minutes. At an easy running pace of 10 km/h it takes about 19. In metres it is 3,100.
What 3.1 kilometres means in practice
3.1 km is the round-number stand-in for a 5K in miles, which is 3.107 miles. If you searched for 3.1 km rather than 3.1 miles, note that the distances are different: 3.1 km is only 1.93 miles.