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Author Topic:   "P-Metric" vs. "LT" vs. " Inflation Pressure"
Associate Member
posted September 14, 2000 10:04 PM     Click Here to See the Profile for Kevhunts   Click Here to Email Kevhunts     Edit/Delete Message Reply w/Quote
Is it acceptable to switch from a P265/70R-16 tire with a "C" load capacity to a LT245/75R-16 tire with an "E" load capacity? If so, would the inflation pressures recommended for the P256's remain the same for the LT245's?

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posted September 15, 2000 11:36 AM           Edit/Delete Message Reply w/Quote
The LT size you are talking about a slightly narrow than the other. As far as inflation pressures and if it's OK to do what you're thinking yes it's fine to do that. Inflation pressure isn't necessarily going to be the same though. I'd say you're going to have to experiment a little probably between 30-35psi (guessing you've got an average Joe type vehicle since you don't mention what it is these tires are going on) and see what type of wear/ride quality you're getting. The LT E load is going to give you a little stiffer ride than what you're accustomed to with the passenger car tire so this is why you'll have to experiment with the pressures. Whatever you do, DO NOT exceed the MAX inflation pressure that will be on the side of the new tires.

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posted September 15, 2000 03:43 PM     Click Here to See the Profile for Kevhunts   Click Here to Email Kevhunts     Edit/Delete Message Reply w/Quote
Thanks for your imput. I have spoken to several different tire stores, Car dealer service dept.'s and tire manufacturers. None of these sources told me I couldn't switch the tires sizes I mentioned. All spoke of the importance of not going lower in load range capacity but, everyone had a different opinion with regards to inflation pressures. Some said to maintain the vehicle's recommended pressures, some said not to inflate less than 50psi regardless of what kind of vehicle they go on, while the rest were suggesting what you recommended. My big concern is running the low 26psi my vehicle manufacturer recommends in a stiffer heavier-duty tire. I don't want them to over heat but, neither do I want my vehicle to become unstable at high speeds. It's already been shown that Ford recommended 26psi on their Explorers because 30psi made them prone to roll over in high speed maneuvers. I guess I'll have to chalk the tires if I get them and experiment, at least I'll get a good contact patch if nothing else.

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posted September 18, 2000 07:48 AM           Edit/Delete Message Reply w/Quote
The load carrying capacity of a tire is related to the amount of air in the tire, and the pressure of the air. The Tire and Rim Association (T&RA) is the US "regulator" of these cpacities. The formulas that are used to calculate the maximum loads on tires due to inflation are different from a P-Metric (P265/70R16) and LT-Metric (LT245/75R16E). The LT-Metric formulas are more conservative and allow for a greater safety margin.

To look at the stated two sizes, to begin there are two issues. First, a P-Metric tire should not have a load range. They are either Light-Load (LL), Standard Load (SL) or Extra-Load (XL). I am assuming they are SL. the second point is that for use on Light Trucks, the load carrying capacity of a P-Metric tire is derated 10% (divided by 1.1). So looking at the tires in question (max loads at inflations):

P265/70R16 at 35 psi: 2185 lbs(2403 lbs/1.1)
LT245/75R16E at 35 psi: 1700 lbs
at 50 psi: 2205 lbs
at 65 psi: 2623 lbs
at 80 lbs: 3042 lbs

So the LT can caryy more ultimate load, but needs more inflation to do it. If you want to carry the same load with the LT245/75R16E tires you need to run them at least 50 psi. Lower than this if you load the vehicle up you could run into problems.

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Associate Member
posted September 18, 2000 06:22 PM     Click Here to See the Profile for Kevhunts   Click Here to Email Kevhunts     Edit/Delete Message Reply w/Quote
Cosmo, I stand corrected. The P-Metric tire I was referring to is indeed SL or Standard Load rated and not "C" load rated. As for inflation pressures, I stumbled across a Load/Inflation table for LT metric tires at www.trucktires.com

It lists tire load limits at various cold inflation pressures. With this information and that of the GAWR for the front and rear of my vehicle, dividing these numbers by 2, I determined that I will need to run 32-35psi up front and 40-41psi in the rear if running on the LT245/75R-16E tires. This is up from 26psi/29psi recommended by my vehicle manufacturer, for my P265/70R-16SL tires and right on track with the information you supplied.
It's disappointing and frightening that the sources I mentioned in my second post were oblivious to any such load/inflation tables and freely give out false information to the general public.

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posted September 19, 2000 11:13 AM     Click Here to See the Profile for Tireman9     Edit/Delete Message Reply w/Quote
re Inflation questions.
It is a fact that all tires loose air at about 1 to 2% per month. Rubber is not an impermiable solid.
For some reason Ford has chosen to ignore this and the fact that a significant portion of car owners seldome if ever check air pressure.
At 1.5% per month it only takes 3 months for a tire at 26psi to drop into the "NOT RECOMMENDED" level according to Tire & Rim Association publications. How often do you check your inflation?
Have you ever had a repair fixed with a plug? Did you know that many tire professionals specifically recommend against using a plug to attempt to repair punctures?

As far as the canned puncture repairs I would strongly recommend against ever using unless you had no spare and only needed to drive far enough , at reduced speeds, to get to a tire store.

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