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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).
 

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So you are saying that with LT tires, under highway +65mph conditions with no load or tow we should use 45psi on the front wheels and 48psi on the rears?
 

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Discussion Starter #3 (Edited)
So you are saying that with LT tires, under highway +65mph conditions with no load or tow we should use 45psi on the front wheels and 48psi on the rears?
From what I understand, the load ratings for various air pressures in the load index tables are the max loads that air pressure can accommodate under 65mph. It seems to accommodate speeds greater than that, we must inflate to the next column's stated air pressure.

However, since 35PSi provides 1900lbs per tire load capacity or 3800lbs per axle, the 35 PSI provides an extra roughly 800lbs of load capacity per axle over the unloaded axle weights so it seems 35PSI should be sufficient for speeds greater than 65mph with no load and up to the speed rating of the tire.
 
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