Paul, I'm not sure specifically about why 15" rims are normally the starting point for LR D tires. I did notice that the highest LR rating you can get with the 5 lug pattern is just over 2000 lbs. If you want more, you have to get the 6 lug pattern.
It may also have to do with the size of brakes required for heavier trailers. If you are pulling a 6000 lb loaded trailer requiring a higher load range, you will need bigger brakes which needs a bigger rim.
Hi Larry, Load Range D tires are available in 14". Yes, you would need to get new wheels rated for the higher pressure. My thoughts were that, if both sizes are rated the same, what advantage is there in going to a 15". I don't really see a reason for adding unstrung weight. I AM going to upgrade my tires before leaving on my summer trip. Trying to make a wise choice.
Paul
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2010 2720SL
Green Cove Springs, Florida
I think we'll have much more choice in tires and rims in the LR D if we switch to 15". If I have room for a 15" rim, I'll get one. Also, some 15" rims are rated at 2600 lbs. I'm not sure if any 14" rims would be rated that high.