Tires: Cadillac Limo tires, cadillac limo, fender skirts

Question
I have a 1996 Cadillac limo that has a tire placard specifying  235/75-15 tires.  In the olden days, Load Range D whitewalls were available.  My problem is I discussed the benefits of extra load and reinforced with a tire manufacturer, and found that 'upgrade' to be 'light', as compared to Load Range C or D (reinforced sidewalls).  This car is heavy, and standard tires allow for too much sway, and feel very overstressed.  I do a lot of freeway driving at 75mph in AZ.  Load Range C has a lower weight carrying capacity than standard tires.   I found one manufacturer making Load Range D, but they use nylon, but there are complaints of flat-spotting, and I let this vehicle sit during the week so I don't want that.  One tire dealer recommended 30x9.5R15 'C', but the weight carrying of these tires is less.  I put on premium 31x10.5-15 'C' on (higher load capacity) and the ride/handling are incredibly good, tire clearance is very tight, and the fender skirts need to be modified to fit.  Of course, I don't get my money back...and are stuck with those tires.  Can you please explain what I should do to select the best 'upgrade tire' possible?

Answer
Bryan,

There is a bit of complexity here - part of which you have a grasp of, and part you don't.  So let's start back at the beginning.

The vehicle placard says the tire size should be 235/75R15.  Does it say "XL" or "SL"?  Does the placard put a letter in front of the tire size - like a "P" or and "LT"?  Once we have this info, we can proceed to answer the path that this follows.

BTW, Load Range C or D doesn't indicate "reinforced sidewalls" per se.  This is an expression of the load carrying capacity for "Light Truck" tires.  That's why we need to know the answers to the 2 questions above.  A "P" type tire doesn't come in "Load Ranges" and we need to know what the vehicle manufacturer specified.