Truck tires are kinda a pain to figure out if you dont have the formulas. I don't know why they keep standard sizes and dont just label them all in metric like automotive has done for the most part as well as ag.

11R24.5 means the tire is a standard profile tubeless tire. 11 is the width in inches. Standard tubeless tires have an aspect ratio of 88% which means an 11R24.5 would be 43.86" tall overall (depending on tread depth) with a radius of 21.93". (11"x0.88=9.68"x2=19.36"+24.5"=43.86"/2=21.93")

285/75R24.5 tires would be 41.33" Overall with a radius of 20.676" (285mmx0.75=213.75mmx2=427.5mm/25.4=16.83"+24.5"=41.33"/2=20.66")

So your overall gain is 2.53" in diameter or 1.27" radius or about a 6% gain.

How does this equate to RPMs? Heres the formulas

Tire diameter x pi (3.1415.........)=Circumference
Mile (5,280')x 12 (to convert to inches)= 63,360" in one mile.
63,360/Circumference=Tire RPM

Sooo
pi*41.33"=129.84"
pi*43.86"=137.79"

63,360"/129.84"=460RPM
63,360"/137.79"=488RPM

Alternatively if you already know your tires diameter and its RPM and know the other tires diameter you can figure the difference percentage wise and apply that to the RPM you know. You know the difference is 6% so if going up multiply by 1.06 if going down multiply by 0.94.

1.06x460=488
0.94x488=460

on a side not 11.00-20 for example is a tube type standard profile tire, these have an aspect ratio of 98%.

If you go to these tires you will need them on all 8 drives you cannot mix and match, technically they want you to keep each pair of tires within 1/8" of each other.

Hope this helps you in the future with all tires.