PSI of LT tire after upgrading from P rated tire

[cj_rezz]

I know this topic has been brought up lots over the years. I’ve read all the posts I can find on the topic on both clubarmada.com and rv.net forums, but still couldn’t come to a conclusion on what PSI to fill my new LT tires (Nokian Rotiva AT Plus) to on the Armada after upgrading from a Passenger, or P, rated tire. So I did a tonne of reading and research and am documenting my findings here for others to reference.

Kal Tire here in Kelowna inflated my new LT tires to 35PSI and said that’s the pressure to use as that’s what’s on the door. Even though the sticker on the door say that’s for a P275/60r20 tire (114 load range), not a LT275/60/r20 (123 load range).

From past reading I wasn’t sure this was entirely true so I started down the rabbit hole!

For reference, I recently went through a weigh scale and the front axle weighed 3,064lbs. Dividing by 2 gives 1532lbs of weight on each front tire, roughly. Using the commonly referenced Load Index Tables, which a copy of can be found here (https://static1.1.sqspcdn.com/static/f/589830/23447320/1378330097907/Discount+Tire+inflation.pdf) , the minimum PSI listed of 35PSI generates a load rating of 1910lbs which should be sufficient to carry that load of 1,532lbs per tire. The rear axle weighed 2,998lbs. Divided by 2 is 1500lbs so, again, the minimum tire pressure of 35lbs provides more than enough load rating at 1,900 lbs. However I have read some comments that this would only be adequate to support 1900 lbs for a maximum of 65mph on a LT tire. To handle greater than 65 MPH (obviously not while towing), you need to add 10PSI. So that would take my 35 PSI to 45. Here’s one source of that information: http://www.trucktrend.com/how-to/towing/1407-understanding-tire-load-ratings . Although at 35PSI the load rating of 1900lbs is a fair ways above and beyond the actual weights of 1,532lbs and 1,500lbs front and rear so I’m not sure how much of a concern this would be. Perhaps if your actual tire weights were right at 1,900lbs, then you would want to add the extra 10Psi for speeds over 65MPH?

When towing our 4,900lb travel trailer, I add about 507lbs of tongue weight and my front truck axle weighs 3,064lbs and rear axle weighs 3,483lbs (with the WD hitch on). 3,064/2=1,532lbs and 3,483/2=1,742lbs. Therefor the 35PSI on a 123 load index tire and a resulting weight rating of 1,900 seems even sufficient for towing this trailer, under 65MPH.

The max front axle weight of an Armada is 3,699. The rear is 4,299. Dividing each of these by two results in 1,850lbs per tire front and 2,150lbs per tire rear. Based on the load index charts, to achieve this max rating of the axle, the front tires would have to have 35PSI (produces 1,900lb capacity) and 45PSI rear (produces 2,280lbs capacity)

However, the load index tables, say that what you should do when converting from a P rated tire to a LT tire, is find the load index rating of the original tire (114) and the PSI the manufacturer recommended (35PSI), then find the load rating for that load index rating and PSI on the LT tire table. In this case, it’s 2,527lbs. Then to use that load rating and with the new tire index table and new tire load index (123) to find the proper PSI for the new tire. It also says when converting from a p-metric tire to a LT tire, you need to reduce the rated load of the P tire by 10%. So in this case the, 2,527lbs-10% = 2,274lbs. A load rating of 2,274lbs on a 123 load index tire means the PSI would need to be 45PSI for both the front and back tires. I also found an article from Toyo Tires about converting from a P rated tire to an LT tire and it agreed with what I determined using this Load Index Table. https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

Using an online tire conversion calculator I found at Tire Pressure Calculator , it suggests I need 48PSI per tire. Pretty close to the 45 above.

I’ve also read in various spots online that the minimum tire pressure you can run on a 80PSI mas pressure tire is 58% of the max pressure or 46.4PSI. Not sure how scientific or true this is or why, in that case, the load index tables would list weight ratings for 123 load index tire with 35PSI, but putting it here for consideration.

So, based on all of this, it seems the best PSI would be somewhere between 45-48 PSI front and back. Perhaps 48 in the rear and 45 in the front. This safely meets the requirements to carry the regular day to day weight of my vehicle at speeds over 65MPH. It also meets the 45PSI the load index charts suggest I should run based on the original tire load index (114) and manufacture recommended PSI (35). It also basically satisfies the minimum tire pressure of 58% of the maximum PSI of 80PSI.

I guess a further step I could take is to inflate to 45-48PSI and do the chaulk test but it seems that is an old test that doesn’t really apply to newer radial tires.

As a side note, I’m really hoping these Nokian Rotiva AT Plus tires work out to be a good compromise between the softer Passenger rated stock tire and a full out 10 ply tire. When my 4,900 lb travel trailer was hooked up to the truck with the WD hitch, the P rated tire sidewalls would flex like crazy when I stood on the bumper and bounced very slightly. A bit unnerving to see, even though the P rated tire was more than capable of handling the weights, technically. Although these new tires are a 10 ply tire, each tire only weighs 40lbs, which is the same weight as the passenger tires I had on before vs 50-60lbs for the same size tire in some other 10 ply tires, so I’m hoping not a big impact on fuel mileage. They also have a 100,000km so hopefully last for a minimum of 5 years of summer driving (I run winter tires in the winter).

[cj_rezz]

Lwiddis wrote:
Why not follow the manufacturer’s specifications?

Unfortunately, unlike other tire manufacturers, Nokian doesn't seem to publicly post load index tables for their tires. I did email them so we'll see if they can provide any information but no reply in the last couple of days unfortunately.

[Fisherman]

Cummins12V98 wrote:

Fisherman wrote:
Easiest way is to use the chalk line across the tires to find optimum PSI. Google it. Short version, take common blackboard chalk and make a couple heavy lines across the tires(all 4). At unloaded weight, you may find 50-55 psi is good for the front, 40-45 for the rear, the chalk will wear off evenly across the tire after 1/4 mile of straight rolling. Walmart parking lot on a dry sunday morning works well. If outside is gone, pressures, too low, if the middle is gone, too high pressure. Unloaded I used to run 52 front, 45 rear. Loaded 60 front, 70 rear.

You towing a TT or 5er?

TT and cargo trailer at different times.

[Grit_dog]

BenK wrote:
Know won't help much...but have to try...

As an example...consider going from a LT class tire to a lower class tire...like P class

Using the current posted logic...would it then make sense to air up your new P class tire using the higher class LT tire's PSI's ?

The P class might blow up...

Agree, not a good experiment.
However I’ve tried that on a few trucks. 1/2 tons with cheesy OE cr@p tires and heavy trailers.
I’ve consistently run P and XL rated tires WELL above their max psi rating on the rear axle. No issues. Like 55-65 psi cold on 35-44psi max tires.
Recommended? Nope
Works? Yup. Makes a squishy tire handle a trailer much better.
Note I don’t do this on the steering axle.

[Grit_dog]

bguy wrote:
The P tires called for 35. When I switched to LT I still used 35. I actually aired up for towing but eventually concluded that the LTs did just fine and rode and wore well at 35.

This answers the original question succinctly and accurately.

[BenK]

Know won't help much...but have to try...

As an example...consider going from a LT class tire to a lower class tire...like P class

Using the current posted logic...would it then make sense to air up your new P class tire using the higher class LT tire's PSI's ?

The P class might blow up...

[bguy]

The P tires called for 35. When I switched to LT I still used 35. I actually aired up for towing but eventually concluded that the LTs did just fine and rode and wore well at 35.

[bguy]

The P tires called for 35. When I switched to LT I still used 35. I actually aired up for towing but eventually concluded that the LTs did just fine and rode and wore well at 35.

[Cummins12V98]

A friend has a 99 Ranger the stock tire size is 245/75R16 "P". He recently had Costco install new Bridgestone tires on his Ranger at just over $800. He mentioned the ride is not as smooth. That got me curious so sure enough they had installed LT245/75R16 "E".

Looked at his door jamb and it said 30psi front and rear. Sure enough they had inflated the tires to 30psi.

I looked up the weight/inflation chart for him and the lowest psi listed was 35 @ 1,710 per tire. Every 5psi the chart adds about 250#. So at 30psi the capacity would be about 1,450# or 2,900# per axle.

I told him the info and he is sticking to 30psi. The tires look to be wearing evenly with about 8,000 miles on them. Most likely he will get 100k no problem.

He said the tires handle good but are stiffer feeling.

[StirCrazy]

are you using a generic table for your tire PSI/Load , do your self a favor and email the manufacture and ask for the load chart for that tire.

I did this for mine, the values are close but they also can give you little tips that are specific for that tire. for mine they recommended not to run the tire under 50psi as at 45PSI they get inconsistent wear.

Steve

[rhagfo]

MitchF150 wrote:
I upgraded to "XL" rated tires for my F150 with 275/55/20 tire size.

They are 50 psi max and have a load rating of #2833 per tire at full psi.

Since the rear axle rating is only #4050 on my F150, I feel it's okay.

I run them at 35 in the rear and 40 up front when not towing. I'll bump them up to 50 on the rear and 45 up front when towing.

Just what seems to work for me on tire wear, comfort and peace of mind..

LRE tires in the same brand/AT were about #20 heavier per tire and cost a heck of a lot more.. Sure, if I had a 25 or 35 series truck that's what I would get, but I just have a wimpy 1/2 ton, so I make due... ha, ha... 🙂

It's just what works for me.. Since you already have the LRE tires, I'd just run them at the different psi settings based on your actual weight like you have already researched.. Overkill? Yes, but you got what you got so just run what works best for you..

Good luck! Mitch

Those sound like a good tire for a 1/2 ton TV.

[MitchF150]

I upgraded to "XL" rated tires for my F150 with 275/55/20 tire size.

They are 50 psi max and have a load rating of #2833 per tire at full psi.

Since the rear axle rating is only #4050 on my F150, I feel it's okay.

I run them at 35 in the rear and 40 up front when not towing. I'll bump them up to 50 on the rear and 45 up front when towing.

Just what seems to work for me on tire wear, comfort and peace of mind..

LRE tires in the same brand/AT were about #20 heavier per tire and cost a heck of a lot more.. Sure, if I had a 25 or 35 series truck that's what I would get, but I just have a wimpy 1/2 ton, so I make due... ha, ha... 🙂

It's just what works for me.. Since you already have the LRE tires, I'd just run them at the different psi settings based on your actual weight like you have already researched.. Overkill? Yes, but you got what you got so just run what works best for you..

Good luck! Mitch

[time2roll]

CapriRacer wrote:
Totally wrong answer!!

The correct answer is: If the numbers in the size are all the same, in order to carry the same load as a P type tire does, an LT type tire has to use 15 psi more (more or less. Precision isn't necessary here!)

I believe my truck with common P tires took 32 psi. My factory options included LT and call for 35 psi on the door sticker. Same size tire.

I would go with the load tables to cover GAWR and add 5 psi not to exceed the sidewall max.

[Cummins12V98]

Fisherman wrote:
Easiest way is to use the chalk line across the tires to find optimum PSI. Google it. Short version, take common blackboard chalk and make a couple heavy lines across the tires(all 4). At unloaded weight, you may find 50-55 psi is good for the front, 40-45 for the rear, the chalk will wear off evenly across the tire after 1/4 mile of straight rolling. Walmart parking lot on a dry sunday morning works well. If outside is gone, pressures, too low, if the middle is gone, too high pressure. Unloaded I used to run 52 front, 45 rear. Loaded 60 front, 70 rear.

You towing a TT or 5er?

[Cummins12V98]

agwill wrote:
Take a chill pill way too much worrying inflate to rated tire pressure listed on sidewall of tire.

Sorry but that is an ill informed suggestion!

[Fisherman]

Easiest way is to use the chalk line across the tires to find optimum PSI. Google it. Short version, take common blackboard chalk and make a couple heavy lines across the tires(all 4). At unloaded weight, you may find 50-55 psi is good for the front, 40-45 for the rear, the chalk will wear off evenly across the tire after 1/4 mile of straight rolling. Walmart parking lot on a dry sunday morning works well. If outside is gone, pressures, too low, if the middle is gone, too high pressure. Unloaded I used to run 52 front, 45 rear. Loaded 60 front, 70 rear.