Understanding Trailer Tire Size

[Supernaut]

My title pretty well explains my question.

I am well familiar with automotive tire sizes and can imagine what the tire would look like by reading the number. But looking at sizes for trailer tires has me scratching my head a bit.

An auto tire as we all know goes like this (width in mm)/(profile height as % of width)/(diameter of rim in inches) or for example P195/65/r15.

But then I see trailer tires listed for example 20.5x8-10. Then its not even consistent, another tire will simply say for example 4.8-12 like I have on my utility trailer. I understand the last number to mean the diameter of the rim but the rest has me confused. Also why there seems to be more than one way to state their size has me more lost. A google seach wasn't much help to me.

Could someone shed some light on the matter? Maybe this could be a sticky thread.

[bobhenry]

The internet knows all.......

http://www.etrailer.com/faq-trailer-tire-sizing-tables.aspx

[Supernaut]

Post by bobhenry » Wed Aug 13, 2014 6:50 am
The internet knows all.......

http://www.etrailer.com/faq-trailer-tir ... ables.aspx

I had already found this chart in my search. By studying the columns one can determine that in a measurement like 20.5x8-10. 20.5 is the overall diameter. 8 I assume is the tire width. That doesn't exactly explain why some measurements don't show over all diameter, or why these apparent different systems exist.

[martymcfly]

I believe that the method to determine sizing has changed. Back in the old days they used sizes like L78-15 or 7.00 16 just for example. All tires were bias ply. Then when I was in the second grade, my teacher said that we all had to learn the metric system, because the whole world was going to change to it. She lied. We only converted some things to the metric system. Tire sizing was one of those that was converted to the metric system. Probably because they are now made in countries that use the metric system exclusively. It seemed like the sizing was changed to metric when radial tires were introduced. Disclaimer here, I have no clue what I am talking about.