[Bulkley]

How do manufacturers go about designing load index? How is a buyer supposed to choose? Here's what puzzles me. Compare these two sizes of Nokian WRG3 SUV.

215/70R16 100H
225/60R17 103H

Note that as the profile decreases and the wheel diameter increases the load index goes up. Why? How will the extra load affect ride?

The example above is not the only one I've seen although it doesn't seem to be universal.

 

[FozzieBalou]

Generally, the load index is how much weight the tires are capable of handling.

It may have to do with the height of the sidewall - a lower profile (and shorter) tire will generally be a bit stiffer in sidewall. Lower profile tires also generally are geared more towards handling and (again) have stiffer sidewalls to limit flex under high-speed cornering.

Or it could just be because of differences in material and side wall construction.

 

[Malvern Sub]

Here is a good article: https://itstillruns.com/difference-tires-102t-tire-sizes-6117752.html
Don't think too much about "why". Load and speed ratings are arrived at by the tire company. Two tires of the same dimensions from two manufacturers (or even from the same manufacturer selling two different product lines) may have different load and speed ratings by virtue of their respective designs.

Specifically, a load rating means that at a given inflation, the load rating is a certain amount. At a lesser inflation, the load rating typically goes down. The load rating is not usually a ride factor but a lower profile tire will typically ride harsher but handle better (it's a trade off).

When I buy tires, I match the tire type to my vehicle and how I drive it and what I use if for. I buy high inflation capacity tire for cars I want to get better gas mileage on like my GEO Metro. For my minivan, I buy tires with high load capacities. I never buy low profiles as they are prone to damage here in the land of crappy roads of Pennsylvania.

Bottom line, get tires that are meant for you and your car, read the reviews, and go for those that are highly rated by the other labels that indicate traction, heat, and wear. Tires ain't cheap and you are stuck with them once you buy them.

Good luck!

 

[Bulkley]

Thanks for the feedback. What I didn't post above is that the tire sizes belong to my two most recent vehicles. The 16 inch wheel is is on a Mitsubishi Outlander and the 17 inch wheel is on my Subaru Crosstrek. Recommended load index for both is 98. It puzzles me that a manufacturer would want to use a higher load indexed tire on the smaller, lighter vehicle.