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).

[Grit_dog]

StirCrazy wrote:

ya the numbers will be pretty close, but if I went for the unloaded weight on my tires, i would be running them at about 35 PSI. but the company said not to run them under 45 so I picked 50 for my unloaded rear PSI to give a bit of room. when I am loaded with the 5th wheel I run 75 PSI, got 80000Km out of the last set doing this so must be good. for loaded I set the tires at 5PSI above the PSI for my actual weight on the chart for the rear.

Steve

All good, but curious what tires and what "company" said 45psi min?
Doesn't make sense. 19.5s need considerably more and I haven't ever seen a LRE tire that was 45psi min.

[StirCrazy]

cj_rezz wrote:

StirCrazy wrote:
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

I am just using generic tables at this point. I did email Nokian a couple of days ago but haven't heard anything back yet. Hopefully I will and this will provide some extra information.

ya the numbers will be pretty close, but if I went for the unloaded weight on my tires, i would be running them at about 35 PSI. but the company said not to run them under 45 so I picked 50 for my unloaded rear PSI to give a bit of room. when I am loaded with the 5th wheel I run 75 PSI, got 80000Km out of the last set doing this so must be good. for loaded I set the tires at 5PSI above the PSI for my actual weight on the chart for the rear.

Steve

[cj_rezz]

CapriRacer wrote:

JIMNLIN wrote:

This information is actually uniform across all tires so we do not have our own chart. I did google it for you though and found this link: http://www.yournexttire.com/trailer-load-inflation-chart/.

The chart is always a good place to start. From there you can fine tune psi that gives you the best ride....best handling and best long term tire wear for a empty truck and a fully loaded truck.

CAUTION: That chart is for ST type tires - and this thread is in "Tow Vehicles"!!

Geez, I didn't even look at the chart they linked to; I just assumed it was a standard chart that I was referencing earlier. They totally linked to the wrong chart!

[CapriRacer]

JIMNLIN wrote:

This information is actually uniform across all tires so we do not have our own chart. I did google it for you though and found this link: http://www.yournexttire.com/trailer-load-inflation-chart/.

The chart is always a good place to start. From there you can fine tune psi that gives you the best ride....best handling and best long term tire wear for a empty truck and a fully loaded truck.

CAUTION: That chart is for ST type tires - and this thread is in "Tow Vehicles"!!

[mowermech]

Years back, I had a Jeep Wrangler Rubicon Unlimited that came from the factory with LT Load Range E tires. Recommended inflation pressure, according to the door jamb sticker? 33 PSI! No way would I ever be crazy enough to inflate to the 80 PSI pressure stated on the sidewall for Max Load!
I made several trips of 1500 miles or more, at 70 MPH in that Jeep. Never had a tire problem.
I spent some time off-roading that Jeep at 5 MPH (or less), with the tires deflated to 12 PSI. No tire problems.
When I bought new tires for the rear of the motorhome, Load Range F tires were not available, I had to buy Load Range G. I told the tire guy to inflate to the LR F specs on the data plate. He argued, but eventually did it my way. No tire problems. They are about to age-out, so I will have to replace them soon. I'm sure we will have the same argument, and he will do it my way again.
I find it interesting that after all the mental gymnastics and number crunching, the OP came up with the same tire pressure as that stated on the data plate, or perhaps a little more, depending on the source. I certainly got a chuckle out of the 80% of rated pressure for LT tires, when Jeep recommended 33 PSI for LT LR E on the Wrangler Rubicon!
Oh, well, it was on the internet, so it MUST be correct, right?

[JIMNLIN]

This information is actually uniform across all tires so we do not have our own chart. I did google it for you though and found this link: http://www.yournexttire.com/trailer-load-inflation-chart/.

The chart is always a good place to start. From there you can fine tune psi that gives you the best ride....best handling and best long term tire wear for a empty truck and a fully loaded truck.

[CALandLIN]

I haven’t seen the correct procedure yet so I’m going to write the way it’s supposed to be done.

First, the replacement tires are required to provide a load capacity equal to or greater than what the OE tires provided as indicated by the certified recommended cold inflation pressures for them, depicted on the vehicle certification label.

Second, the vehicle manufacturer already accounted for the 10% decrease in load capacity for “P” tires with their recommended cold inflation pressures for the OE tires.

Third, you’ll need a load inflation chart for the replacement tires. With it you can set new recommend cold inflation pressures for the replacement tires with an inflation pressure that will meet the minimum requirements. Anything above your new recommended cold inflation pressures is optional all the way to tire sidewall max.

[cj_rezz]

FYI, I received the following email back from Nokian:

Thank you for contacting Nokian Tyres.

This information is actually uniform across all tires so we do not have our own chart. I did google it for you though and found this link: http://www.yournexttire.com/trailer-load-inflation-chart/.

If your tire size does not appear on that chart you can just google "LT load and tire pressure chart" and you should be able to find it.

Let us know if you have any questions.

[JIMNLIN]

cj_rezz wrote:

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.

Thanks for this info. Based in generic load index tables, 35PSI would be fine for regular daily driving for me too. It would even be fine to support the actual weight put on the rear axle when towing my specific travel trailer. However, I would need 45PSI to obtain the max rear gross axel weight rating though if I was towing something heavier.

I'm curious but what psi were you using in those P tires.

P metric tires max capacity comes at 35 psi even thought the sidewall may say 44 psi.
Bumping them on up to 44 psi stiffens the sidewalls for hauling or towing conditions.

If you notice the P metric sticker pressure is at 35 psi for a specific load. The same size LT will need another 15 psi or so for it to meet the same load.

However your hard part is finding a psi compromise for handling/ride/less rolling resistance/etc from those heavy E tires.

[cj_rezz]

blt2ski wrote:
The generic tables you have found, work for ALL brands of tires. As long as you compare size for size with in brands. That is a federal standard.

No I am not surprised that a 10 ply tires, holding x lbs, assuming with in specs of a P metric, is the same max load.

I personally on one of my SW 3500 years ago, found no difference in handling in an 8 vs 10 ply when both were at 60 or less lbs of psi, same loads etc. Only time the LR E came into play, was when I put 2 yds of gravel in the bed, then 80 vs 60 lbs of air did help.

For most folks, getting the lowest load rating that is above what they will be at.

Marty

Thanks! Makes sense.

[blt2ski]

The generic tables you have found, work for ALL brands of tires. As long as you compare size for size with in brands. That is a federal standard.

No I am not surprised that a 10 ply tires, holding x lbs, assuming with in specs of a P metric, is the same max load.

I personally on one of my SW 3500 years ago, found no difference in handling in an 8 vs 10 ply when both were at 60 or less lbs of psi, same loads etc. Only time the LR E came into play, was when I put 2 yds of gravel in the bed, then 80 vs 60 lbs of air did help.

For most folks, getting the lowest load rating that is above what they will be at.

Marty

[cj_rezz]

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.

Thanks for this info. Based in generic load index tables, 35PSI would be fine for regular daily driving for me too. It would even be fine to support the actual weight put on the rear axle when towing my specific travel trailer. However, I would need 45PSI to obtain the max rear gross axel weight rating though if I was towing something heavier.

[cj_rezz]

StirCrazy wrote:
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

I am just using generic tables at this point. I did email Nokian a couple of days ago but haven't heard anything back yet. Hopefully I will and this will provide some extra information.

[cj_rezz]

Grit dog wrote:

cj_rezz wrote:

rhagfo wrote:

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

This advice is extremely incorrect in the OP case! The OP has the correct inflation for his tires figured out. fully loaded 35 front 45 rear. I would run 35 rear empty.

Would you run 35 in the rear when not towing just for the comfort or more due to likely more even tread wear across the width of the tire? If it's just for ride comfort, the ride is not bad at all with 45 in the front and rear but definitely don't want to cause any uneven wear with too much tire pressure.

It’s sort of a balance between pressure, tire vs rim width and tire construction.
Some combos cause the tire to balloon a bit more than others. Others are happy tuning higher than needed pressure without affecting treadwear.
Generally if rim width is well matched to tire width less potential for ballooning or the opposite with too wide of rims.

Interesting. Thanks for that insight.