Tire size meaning??

Browsing the Mich website for Defenders in 265-70-17, I see 115T and 121R as options.

What do 115T and 121R mean? Are they a rim width designation?

 

Probably the load range.

 

Load and speed rating:

https://m.tirerack.com/tires/tiretech/techpage.jsp?techid=35

 

Looks like speed rating goes after the ratio and before the rim size.

 

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

Load and speed ratings.

 

https://www.tirebuyer.com/education...ratings-load-indexes-and-service-descriptions

 

I we were both right it seems.

 

Thanks guys. I figured the load ratings were simply P or E, etc. I guess not.

 

Load ranges were introduced to replace the ply rating system. C=6, D=8, E=10 and so on. In the old days the more plys the higher the load capacity. You literally counted the number of plys. Modern tires are much different that it doesn't matter the number because there are so many different materials that are used that have different strengths. That is why they came out with the load index which is a chart that correlates that number to a weight capacity. Typically the load index is combined with a speed rating. When combined you can run the tire at the maximum speed for the rating at the maximum load at the max tire pressure for 1 hour safely.

Now the P on the other hand is a different designator. That is the tire classification. In simple terms you have P and LT designations that preface a size. P for passenger car and LT for Light truck. You will see these in the same size for some tires. LTs typically have a higher load capacity than a P, and in some cases you can see that in the load index after the size. Not all Ps will have a load index. But, it doesn't end there because there are P metric and metric tires. If the size does not start with a P or an LT it is simply a metric tire. Metric tires are common in Europe and will usually have a higher load capacity than a P metric. There are tires that are available in P metric and metric version of the same tire. I don't know any that are made in all 3 variants, but there is a possibility.

What does all this mean? In reality nothing. Only a tire geek understands this granularity. Usually when you see tires like the two Michelins posted is that one was an OEM tire. Car manufactures have ridiculous specs that tire makers will accommodate by making a very similar tire just to get OEM.

 

Wow. Great info @Sungod