Posted by ChicagoMike on April 09, 03 at 20:54:23:
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. See my previous
posting about underinflation for many of the facts and figures used in this posting.
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.
From my previous underinflation posting I have indicated that my 2dr 4wd 1999 Chevy Tahoe, which
came with P245/75R16 tires, requires a tire with a load rating greater than 1963 lbs.
Also from my previous underinflation posting I indicated that 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.
Finally, from my previous underinflation posting I indicated that 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.
Doing some calculations shows that a load rating of 2030 lbs at 45 psi is 3.4% greater than 1963 lbs,
while 2205 lbs at 50 psi is 12.3% greater, and 2335 lbs at 55 psi is 19.0% greater, and 2480 lbs at 60
psi is 26.3% greater, and 2623 lbs at 65 psi is 33.6% greater, and 2765 lbs at 70 psi is 40.9% greater, and 2900 lbs at 75 psi is 47.7% greater, and 3042 lbs at 80 psi is 55.0% greater.
(1) If an LT245/75R16 LRC tire with a load rating of 2030 lbs at 45 psi exceeds the minimum required
load rating of 1963 lbs by 3.4% then why would I need an LRD or LRE rated tire for my Tahoe? My axles would fail before the tires would.
(2) Providing a load rating for an LT245/75R16 tire that is 3.4% or even 12.3% greater than the
minimum required 1963 lbs sounds reasonable. However, is providing a load rating that is 19.0% to 55.0% greater acceptable or would it be considered a sign of an overinflated tire?
(3) At what load rating and pressure (less than the tires maximum rated pressure) would an LRC or LRD or LRE tire be considered overinflated and why for my Tahoe?
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.
Doing some calculations shows that a load rating of 2100 lbs at 40 psi is 7.0% greater than 1963 lbs,
while 2280 lbs at 45 psi is 16.1% greater, and 2470 lbs at 50 psi is 25.8% greater.
(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?