Tires: tires and rim sizes, proper inflation pressure, ford f250 super duty

Question
I have a 2000 Ford f250 Super duty diesel truck and it needs new tires. The tires on there now are 285/75/16 the look to possibly be a little larger than stock but I have no owners manual with the truck. I was looking at tires and saw a pair of 36" x 12.5" x 6.5" will these fit? I have been looking for tire calculators onluine and cant find anything. Also my truck weighs more than a ton and i typically carry 1000 pounds in the back can i use a load range "c" or do I need a "D" or "E"

Answer
Jeremy,

It's a little hard to tell what the original tires were on your truck because there are so many versions, but if you'll look at the driver's doorpost, I think you will find the vehicle placard and that will tell you the original tire size and the proper inflation pressure for that size.

My best guess is LT235/85R16.

AND!!!

I think you meant 36" X 12.50 R 16.5LT , the key is 16.5 not 6.5.   And these will not fit on your rims.