F 250 Tire load rating.

You have to go size by size to determine. I am going to go with 265/70R17 vs 265/70R18, and then 265/70R17 vs 265/65R18. All numbers are from BF Goodrich KO2 Tires.

Going from 17 to 18, you go from 3195lb (17") to 3525lb (18") but you also gain 1 inch in overall tire height, effectively reducing your gear ratio slightly.

Now, keeping it even sizes, comparing 265/70R17 and 265/65R18, you go from 3195lb (17") to 3305lb (18"), and you lose (not loose, that refers to lug nuts that are not tight) .1" of overall tire height.

Load rating has to do more with the sidewall profile and width than it does wheel diameter. With smaller diameter wheels, you can get a higher profile tire in the wheel well - that's all. As a general rule, you will LOOSE load capacity as you increase the wheel diameter because you have to use a lower profile tire to keep the outside diameter the same. Also, different manufacturers may have a slightly different rating for the same size tire in the same load range. Here's a good place to look at different options and then see what tires are available in the size you choose:

Tire size calculator

Note that, as you change wheel sizes and tire profiles, the outside diameter of the tire will change, causing changes in final gear ratios and height of the vehicle - very important when towing.

Rob

I have seen that there can be different load ratings for the same letter rating.
I see no indication that the size of the wheel has any bearing on the load rating of the tire.