Ok, so there's minor difference's to the 2 types. The D has cross woven cords and is considered a durable tire that withstands uneven surfaces better than a R rating,sidewalls are stronger with a lower speed rating. there designed to withstand the UV rays also.
The R tire is acceptable to use in place of a D rated tire. Some of the benefits are a cooler running tire more design for highway use, with cords that are parallel instead of a woven design, improves gas mileage and are typically less expensive than a D tire .
So I bought Goodyear marathon ST20575R 14s and expect no difference between the old 20575D14s.
