Posted by ChicagoMike on April 09, 03 at 20:51:12:
When are LT tires underinflated or overinflated?
I'm planning on replacing my existing P245/75R16 Michelin LTX M/S tires with LT265/75R16 LRC Bridgestone Dueler AT Revo tires. While doing research on getting load range (LR) C or D or E tires,
questions of when are LT tires considered underinflated or overinflated came up. I'll discuss the questions of overinflation in another posting otherwise this will be a very long message.
For the purposes of this discussion I'll compare P245/75R16 tires with LT245/75R16 tires since I have
complete load rating information for these size tires and the LT tires are available in load ranges C, D
and E. The principles should apply to LT265/75R16 tires.
To start with, my 2 dr 4wd 1999 Chevy Tahoe has a Gross Axle Weight Rating (GAWR) of 3,925 lbs
for the front and 3,750 lbs for the rear. The GAWR is the maximum weight to be carried by a single axle. The total load on an axle should never exceed its GAWR. Assuming that the tires on an axle should have a combined load rating greater than the axle's GAWR, each tire on my Tahoe should have a load rating greater than 1,963 lbs (3,925 lbs/2).
The vehicle certification label on my driver's door says to use P245/75R16 tires at 35 psi. According to
the Goodyear service load and inflation table for passenger car tires used on light trucks, a P245/75R16 tire has a maximum load rating of 2,065 lbs at a maximum pressure of 35 psi. This load rating is already reduced 9 percent from the tires actual maximum load rating of 2,271 lbs at 35 psi.
Also according to the table, the tires maximum load rating is 1,975 lbs at 32 psi and 1,884 lbs at 29 psi.
According to the Bridgestone tire website for LT tires, the load ratings for LRC, LRD and LRE are the
same at pressures less than the maximum rated pressure for the particular load range. The maximum pressure ratings are 50 psi for LRC, 65 psi for LRD and 80 psi for LRE. For example, LRC, LRD and LRE tires all have the same load rating at 45 psi while LRD and LRE tires have the same load rating at 60 psi.
According to both the Goodyear service load and inflation table for light truck metric tires and the
Michelin load/inflation charts, the load ratings for an LT245/75R16 tire are as follows: 1700 lbs at 35 psi, 1865 lbs at 40 psi, 2030 lbs at 45 psi, 2205 lbs(C) at 50 psi, 2335 lbs at 55 psi, 2480 lbs at 60 psi,
2623 lbs(D) at 65 psi, 2765 lbs at 70 psi, 2900 lbs at 75 psi and 3042 lbs(E) at 80 psi.
An oem option for my Tahoe was the LT245/75R16 LRC tire. From a Firestone tire dealer's computer
database, I found that Chevrolet had specified that this tire should be inflated to 45 psi. This makes
sense since the load rating at 45 psi is 2030 lbs which is greater than 1963 lbs.
This also indicates to me that an LT245/75 tire on my Tahoe, whether LRC or LRD or LRE, at 40 psi
(1865 lbs rating) or at 35 psi (1700 lbs) would be underinflated because it has a load rating less than
1963 lbs.
Four questions about underinflation can now be asked:
(1) Is a tires load rating at a particular psi the only criteria for underinflation or are there other factors that can be considered underinflation?
For an LRC tire, a specified pressure of 45 psi is 90 percent of the maximum 50 psi. For an LRD tire, a
specified pressure of 45 psi is 69 percent of the maximum 65 psi. For an LRE tire, a specified pressure of 45 psi is 56 percent of the maximum 80 psi.
(2) Filling an LT245/75R16 LRC tire on my Tahoe to 90 percent of its maximum pressure sounds reasonable. However, is an LRD tire on my Tahoe filled to 69 percent of its maximum pressure or an LRE tire on my Tahoe filled to 56 percent of its maximum pressure acceptable or would they be considered underinflated?
(3) If the LRD and/or LRE tires on my Tahoe would be considered underinflated at 45 psi, then what would be acceptable minimum pressures and why?
For the LT 265/75R16 LRC tires I'm considering, the load ratings are 1910 lbs at 35 psi, 2100 lbs at 40 psi, 2280 lbs at 45 psi and 2470 lbs at 50 psi.
(4) Should I fill these tires to 40 psi or 45 psi for a safe load rating (both greater than 1963 lbs) and a
comfortable ride?
My questions about overinflated tires will be asked in another posting.