Why is it that 17" tires carry the lowest load ratings? There are 2 tires, toyo mt and nitto dura grappler, that have load ratings that i can use. Why is this? Why do I need to step up to an 18" wheel to get decent load ratings? I emailed tire companies with this question and got no logical response other than, you need to get a larger wheel size. I haul a heavy truck camper and need that extra assurance.