3.142 = pi
60 = conversion from minutes to hours
1000 = conversion from meters to kilometers
1 rotation of tire is the distance traveled in 1 revolution. To calculate the distance of 1 rotation you need to know the circumfrence of the tire, which is "Diameter x pi". So a 30 inch tire would have a circumfrence of 30 x 0.0254 x 3.142 = 2.394 meters
So in 1 wheel rotation the tire crosses a distance of 2.394 meters.
so if you mutiply 2.394 x Wheel RPM = you get wheel speed in meters / minutes
hence we need the 60/1000 conversion for kilometers / hour
I assume you are trying to find out your gear ratio on the MUTT, here they are
1 gear 5.712:1
2 gear 3.179:1
3 gear 1.674:1
4 gear 1.0:1
reverse 7.497:1
MUTT's differential ratio is 4.86:1
4x4 Transmission ratio is 1:1
For other vehicles just google the model / type of gearbox / diffs and you will get the ratio details. Typically all 4th Gears have a ratio of 1:1