The Squealing Tire

Tire Talk from the Experts at Discount Tire Company.

The Squealing Tire - Tire Talk from the Experts at Discount Tire Company.

LT Tires and How They Work

Ever wonder when reading tire reviews with complaints about “squishy” or “slow responding” traits….Is the tire to blame or was it the size the user decided to run? Did this driver upgrade from a stock tire with a 80psi max pressure to a larger tire with a 65psi max?

These questions pop into my mind time and time again and the answer is not always in the consumer review.

With LT-Metric Sizing and LT-Flotation Sizing being the most popular LT Sizing you’ll find on today’s trucks, and by today’s trucks we mean SRW 1 ton, 1/2 ton and 3/4 ton, we decided to take some time to explain the LT Tire System and focus on load carrying capacity. In this write-up we will define LT Tire ratings including LOAD RANGE and LOAD INDEX and explain in detail how the two work in conjunction with each other to determine load carrying capacity. We’ve also included an TL tire load index chart. This topic can be somewhat confusing so bear with us to the end and if you have any questions please feel free to post them here.

Below is an example of LT-Metric Sizing and LT-Flotation Sizing. We’ve broken down the characters and numbers in each size to explain what they mean, which most of you already know.

LT-Metric Sizing
LT265/75R16/E 123R

LT = Light Truck Designation
265 = Section Width (mm)
75 = Aspect Ratio
R = Radial Construction (most common construction used today)
16 = Rim Diameter (inches)
E = Ply Rating
123 = Load Index
R (after load index) = Speed Rating

LT-Flotation Sizing
31×10.50R15LT/C 109S

31 = Tire Diameter (inches)
10.50 = Section Width (inches)
R = Radial Construction (most common construction used today)
15 = Rim Diameter (inches)
LT = Light Truck Designation
C = Ply Rating
109 = Load Index
S = Speed Rating

As mentioned above, we want to specifically look at the Load Index and Load Range. Without the Load Range and Load Index, you cannot determine the maximum load carrying capacity of the tire.

Q: What is Ply Rating?
A: Ply Rating: This letter indicates the load carrying capacity of the tire in terms of its construction. A “C” indicates the tire has a 6-ply load carrying capacity. The tire is not actually built with 6 plies, but contains one or two plies of equivalent strength. A “D” is an 8-ply rating, and an “E” is a 10-ply rating. If there is no letter, the tire has a standard 4-ply rating.

Q: What is LOAD INDEX?
A: LOAD INDEX is an assigned number ranging from 0 to 279 that corresponds to the LOAD-CARRYING CAPACITY of a tire.

Like most of you know, Load Range describes the ply rating of the tire. It’s important to remember that these are ONLY RATINGS and not the actual number of ply’s like it used to be back when tires were made of cotton belts (bias). Below are the most common Load Ranges you’ll see on today’s trucks.

C = 6 ply rated
D = 8 ply rated
E = 10 ply rated

Now here’s where it can get a bit confusing. It’s commonly assumed that C rated tires hold a maximum pressure of 50 PSI, that D rated tires hold a maximum pressure of 65 PSI, and E rated tires hold a maximum pressure of 80 PSI. This information is correct but only applies to a certain tire size range, or rather, section width.

Here’s what you need to focus on.

LT tires that are 295mm (LT-Metric) in section width or 11.50″ in section width (LT-Flotation) and narrower, will carry the commonly associated maximum pressure.

C = 6 ply rated and hold a maximum pressure of 50 PSI
D = 8 ply rated and hold a maximum pressure of 65 PSI
E = 10 ply rated and hold a maximum pressure of 80 PSI

Not so tricky I hope. So you’re probably thinking to yourselves, what about the other sizes? And I’ll tell you…

LT tires that are 305mm (LT-Metric) in section width or 12″ (LT-Flotation) and wider, still carry the same ply rating/Load Range but hold a different maximum inflation pressure. The maximum inflation pressure on these wider sizes does not require as much pressure to achieve the same load carrying capacity. The new inflation pressures on these wider sizes are as follows:

C = 6 ply rated and hold a maximum pressure of 35 PSI
D = 8 ply rated and hold a maximum pressure of 50 PSI
E = 10 ply rated and hold a maximum pressure of 65 PSI

Now what about Load Index? Like mentioned previously, Load Index is an assigned number that corresponds to the load carrying capacity (more specifically, at the maximum pressure). The assigned number or Load Index can be seen on the chart below.

  • Please note that this chart only goes up to 150 as most LT truck tires today do not exceed 128.

Load1.jpg

Load2.jpg

So lets take the Load Index 123 for example. This Load Index tells us that the corresponding load carrying capacity is 3,417 lbs at maximum pressure. Now it doesn’t show what the maximum pressure is so we now have to go look at the Load Range on the tire we are looking at.

Taking the example of LT-Metric Sizing listed earlier (LT265/75R16/E 123R), we know the tire is 10 ply rated and is narrower than 295mm in section width which tells us that the maximum inflation pressure is 80 PSI. By using the Load Index chart we know the 123 Load Index carries 3,417 lbs at maximum pressure. So from what we have discussed, we now know that the LT265/75R16/E 123R tire carries a maximum load of 3,417 lbs @ 80 PSI.

Now taking the example of LT-Flotation Sizing listed earlier (31×10.50R15LT/C 109S), we know the tire is 6 ply rated and is narrower than 11.50″ in section width which tells us that the maximum inflation pressure is 50 PSI. By using the Load Index chart we know the 109 Load Index carries 2,271 lbs at maximum pressure. With this information we now know the 31×10.50R15LT/C 109S tire carries a maximum load of 2,271 lbs @ 50 PSI.

A lot of people think that Load Range E tires will always carry more weight than the lower ply rated/Load Range tires. This is not always the case. Let me show you an example.

A common OE (original equipment) LT tire size is LT245/70R17/E 119S. By using what we have learned above we know that this LT tire size carries 2,998 lbs @ 80 PSI. Now lets look at an LT315/70R17/D 121T which is a common plus size LT tire. Again, from what we have learned above we know this tire carries 3,197 lbs @ 50 PSI. So even though the OE tire we used for an example is E rated with a maximum pressure of 80 PSI, the plus size D rated tire carries more weight at a lower pressure. In other words, LOAD RANGE E doesn’t always mean the tire can carry more weight at maximum pressure when compared to LOAD RANGE D at maximum pressure.

 

Matt Johnson
Discount Tire Co.

Matt JohnsonAbout The Author: Matt Johnson (13 Posts)

I've been a Discount Tire Trusted Expert for 9 years now and an automotive enthusiast for most of my life. Whether it’s driven on the track, off-highway, or on the street, I’m interested! I particularly enjoy crunching numbers and finding truth in the technical details. Wheels and tires may seem like a simple thing but those that have tried to plus size any application know there’s a lot more to it than just rubber, steel and aluminum. Click the Author Profile Icon below to read more.