What does NHS mean?
Many people think that when they see 23x8.50-12 NHS on the side of their lawn tractor tire that it is part of the tire size. NHS stands for Non-Highway service. This means the tire is not rated to be used at highway speeds.
Let's take an example lawn tractor tire size and break it down. In our example our lawn tractor will have a tire size of 15x6.00-6.
The first number in our lawn tractor tire size is 15. This number refers to the overall diameter of the tire or the tire height. In this example our tire will be roughly 15" inches tall.
The second number in our lawn tractor tire size is 6. This number refers to the width of the tire. In this example our tire will be roughly 6" inches wide. This doesn't mean the tire mounts on a 6" wide wheel, normally the tire width will be wider then rim width.
The third number in our lawn tractor tire size is 6. This number refers to the rim diameter or the size of the hole in the middle of the tire. In this example our tire will mount on a rim with a 6" diameter.
Let's do another example.
If our lawn tractor tire size was 18x8.50-8 NHS. We know right away that our tire is not rated for highway service because the letters NHS are on the tire. The first number is 18 which means the tire is roughly 18" in diameter. The next number is 8.50 which means the tire is roughly 8.5 inches wide and the final number is 8 which means the tire mounts on a wheel with an 8" diameter.
Now that we have mastered the tire sizing, lets look at another piece of information which is on the side of your lawn tractor tire. Ply rating or load range refers to the carrying capacity of the tire based on the number of plys of rubber that make up the tire. Generally speaking the higher the ply rating or load range the greater the number of plys that make up the tire and the more weight the tire can carry. Different tire manufacturers use different terms to relay the load range of the tire. Some simply use terms like 2 ply, 4 ply, 6 ply, etc. Some use load ranges or abbreviations like LRA, LRB, LRC, etc. Below is a conversion chart to help with determining the ply rating of your tire.
Load Range A, LRA, 2 Ply
Load Range B, LRB, 4 Ply
Load Range C, LRC, 6 Ply
Load Range D, LRD, 8 Ply
Load Range E, LRE, 10 Ply
Load Range F, LRF, 12 Ply
If we check the sidewall of our lawn tractor tire and we see 23x8.50-12 NHS LRB. We know that the tire is 23 inches tall, 8.50 inches wide, mounts on a 12 inch diameter wheel, is not rated for highway use and is a 4 ply tire.
As ply ratings increase, we increase the carrying capacity of the tire. For example a Carlisle Turf Save Tire in a 18x8.50-8 is available in a 4 ply and an 8 ply. The 4 ply version is rated to carry 815lbs of weight per tire, the 8 ply is rated to carry 1220 lbs of weight per tire.
I hope this helps make reading your lawn tractor tire size easier and helps you make better decisions on your lawn tractor tire purchases.