Pace per kilometre and pace per mile describe the same speed over different unit distances. The international mile is exactly 1,609.344 metres, so the conversion ratio is fixed and does not need live data or a location setting.
Use the international mile
One mile equals exactly 1.609344 kilometres. Multiplying exact seconds per kilometre by 1.609344 gives seconds per mile; dividing seconds per mile by 1.609344 gives seconds per kilometre. PaceCalc represents the ratio with integers, so the conversion itself adds no floating-point error.
Check a 5:00 per kilometre pace
Five minutes per kilometre is 300 seconds. Multiplying by 1.609344 gives 482.8032 seconds per mile: 8 minutes and 2.8032 seconds, displayed as 8:03 per mile. The exact value, not 8:03, remains the basis for finish times and splits.
Keep pace and speed distinct
Pace is time per unit distance, so a smaller pace is faster. Speed is distance per unit time, so a larger speed is faster. PaceCalc shows both km/h and mph from the canonical distance-time ratio; it never converts a rounded pace display back into speed.