Help with tyre size meanings

[aesmith]

Hi,

I'm familiar with the size markings for road tyres. For example our Landrover has 235/85 16 (omitting load, construction and speed rating). Meaning 235mm wide, height is 85% of width, and 16" rim diameter. If omitted, aspect ratio is 80%.

Tractor tyres seem to have either fewer or more measurements. For example 7-16 is presumably 7" wide on 16" rim. Height is what, 80% of the width like a road tyre, or something else?

Turf and some industrial have more sizing figures, for example 27 x 8.5-15. I can guess that's 15" rim diameter utility what does the 27 indicate?

See a couple of examples of agriculture and equipment turf or industrial tyres ..

 

[TractorTech]

The 27 indicates that it is 27" in diameter.

 

[aesmith]

Cheers. Can I ask how to get diameter from the other formats, for example 11.2-24?

Trying to square my workings with fact that one example quotes around 10% slower on 15-19.5 compared to 11.2-24.

 

[TractorTech]

Unfortunately, for R1, 2, 3 and 4 tires, you need to either find the radius or diameter information on a couple of manufacturers to get that information or physically measure the tire you are looking to use.

Here is a guide book that can come close for most brands:

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://commercial.bridgestone.com/content/dam/dcoe/commercial/global/pdfs/ag-databook-2025.pdf&ved=2ahUKEwifupa45tuSAxX4OUQIHX1TJcIQFnoECF0QAQ&usg=AOvVaw3ItQwhR2yi45zmQxkaEbIv

 

[rScotty]

Unfortunately, for R1, 2, 3 and 4 tires, you need to either find the radius or diameter information on a couple of manufacturers to get that information or physically measure the tire you are looking to use.

Here is a guide book that can come close for most brands:

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://commercial.bridgestone.com/content/dam/dcoe/commercial/global/pdfs/ag-databook-2025.pdf&ved=2ahUKEwifupa45tuSAxX4OUQIHX1TJcIQFnoECF0QAQ&usg=AOvVaw3ItQwhR2yi45zmQxkaEbIv

That's what I do as well. As near as I can tell, the offroad/tractor/industrial tires makers don't have a standard for describing their tires that works across all manufacturers. Use the brand name and then go online to find their annual tech specification booklet. Sometimes you have to search to find it.

Since tires are a flattened oval rather than a true circle you have to use a measurement called the "loaded rolling circumference" (LRC, or just RC) to do calculations for speed or gearing. That's why rolling circumference is always spec'd in the tire tech booklet.

To measure RC on an unknown tire or unknown load, put two wet paint blobs on the tire, drive in a straight line, and then average the distance between marks.
Enjoy,
rScotty

 

[RandyT]

Something to keep is mind is most tires are now radial tires which are described with the overall diameter, width aspect on automotive tires, or overall width on industrial/ag tires, and rim size. The older bia ply tires are only described with overall width and the rim size. Back when vehicles used bias ply tires they were also described using width and rim size with no overall diameter listed.

 

[404nouser]

Without a cap ply a bias tire can't get much under 90% aspect ratio and rim width affects profile.
But low pressure tires can be molded a little further than the 90% when using an appropriate wheel.