Not to nitpick but while the 315/70R17 is S rated, floatation sizes like 35x12.50 or 37x12.50 are not S rated. They are R rated for 106MPH. I have yet to see a floatation size tire with a speed rating over 106MPH. I believe this has to do with differences in the construction. So @oxfordraptor is still technically correct.
Lol. Bottom line...look at the speed rating on the actual tires you're running to determine the speed rating. People call the LT315 tires 35's all the time even though they aren't technically.