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Old 04-18-2011, 03:23 PM
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Default 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 an 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 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 LOAD RANGE and LOAD INDEX and explain in detail how the two work in conjunction with each other to determine load carrying capacity. This topic can be some what 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 = Load Range
123 = Load Index
R (after load index) = Speed Rating


LT-Flotation Sizing
31x10.50R15LT/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 = Load Range
109 = Load Index
S = Speed Rating
Like mentioned above, we want to specifically look at the Load Index and Load Range. Without the Load Range and Load Index, you can not determine the maximum load carrying capacity of the tire.

Q: What is LOAD RANGE?
A: LOAD RANGE (also known as the load range description) is the load range rating found on the sidewall of LT tires. It's used to determine the maximum load carrying capacity of the tire UNDER A DEFINED MAXIMUM INFLATION PRESSURE.

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 12.50" (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 lose 15 PSI from the maximum inflation pressures above. 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



So lets take the Load Index 123 for example. This Load Index tells us that the corresponding load carrying capacity is 3417lbs 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 3417lbs at maximum pressure. So from what we have discussed, we now know that the LT265/75R16/E 123R tire carries a maximum load of 3417lbs. @ 80 PSI.

Now taking the example of LT-Flotation Sizing listed earlier (31x10.50R15LT/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 2271lbs at maximum pressure. With this information we now know the 31x10.50R15LT/C 109S tire carries a maximum load of 2271lbs @ 50 PSI.

Here's where it get's fun.

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 2998lbs @ 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 3197lbs @ 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.

Here's where your application and uses come in.

What does this mean to you if you are upgrading from the stock E Load Rated tire with an 80psi max to a new, larger, E rated tire with a 65psi max What if your upgrading to a larger size that is D load rated?

If you do not tow much it may not matter as long as your new tire carries equal to, if not a higher, load index number. The new tire will still be able to carry at least the same weight at 65psi as your stock tire would at 80psi. The only real difference is the way the vehicle may handle under load. If you are accustomed to towing with 80 psi in your tires and now you are only able to tow with 65psi, your vehicles response can feel squishy or lazy and you can experience a bit more "roll" as the vehicles weight shifts in turns.

How do you tell if the E Load Rated tire you are looking to buy has a 65psi or 80psi max?

On our website we indicate all 80psi E Load Rated tires as "E1" and all 65psi E Load Rated tires as "E2" in the tire size. Same thing goes for "D" and "C" Load Rated tires.
Examples:

E1 & E2: Nitto Trail Grappler M/T tires listed by size
D1 & D2: Nitto Terra Grappler AT tires listed by size
C1 & C2: BFG All-Terrain T/A KO tires listed by size
I hope this information helps.

-Matt
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