tire size question

Trying to get a tube for an older tire. Have a question about modern sizing. What's the difference between
KR14/15 and MR14/15 sizing? thanks

kr is smaller
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thanks

On a tire related note..... We have 4-5 miles of roadways to maintain, along with the usual projects requiring gravel fill. We have a gravel pit over on the back side of the farm but it's a bit over a mile from the farmstead. Although we have expended a fair amount of time and effort building a road to the pit, a good deal of it crosses low ground and wet spots to a degree that we have to limit our hauling to dry times (typically haying season) when there are a zillion other things to do. We have two single axle dump trucks one with a 6 yard box, the other an eight. I see some large dairy farmers in neighboring counties using really wide single flotation tires in place of the duals on their semi trucks/trailers for hauling out of the field to over the road with corn in the fall and thought wow; wonder how pricey that would be to put tires like that on one of our dump trucks. I would extend our ability to haul gravel to a two/three month window, rather than only a month, some years less. I'm thinking that these "big guys" would have to have cast-off tires, no longer roadable that would work well for our purposes. Anyone here with any idea where a person might find the necessary wheels, or for that matter if the idea is even practical?