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John E Davies

Tire TECH: Max load vs tire pressure charts for non-standard type and sizing

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These are from Toyo but the specs are very similar between tire manufacturers. A Nitto owner on a forum called Nitto customer service to ask about charts and was told to use the Toyo ones.

 

If you run the OEM tire type and size, use the chart glued inside your driver door opening or the info inside your Owners Manual.

 

If you have switched to a different size, or from a P rated (passenger car) tire to a LT rated (truck) tire, you need to study the charts to figure out the pressure you should use. LT tires require more pressure to carry the same load as a P rated tire. It doesn’t seem to make sense, but it’s true.

 

https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

 

You do not need to know your actual axle scale weights, though that would be useful information for loading your rig and figuring tongue weight. You can calculate, using the charts, the different load capabilities and figure out a good target pressure to run.

 

Example:

 

My 2013 Land Cruiser came with P285/60R18 car tires (31” diameter) at 33 psi front and rear. The truck has a nearly 50/50 front to rear weight balance, unloaded. The Gross Vehicle Weight Rating is 7385 pounds. The P- Metric chart from Toyo shows a max load of 2513 pounds per tire (5026 per axle) for this size at Toyota’s recommended 33 psi. So that is the “target load” that they thought was appropriate. These tires were lightweight for better fuel economy and very squooshy for passenger comfort. They were also fragile and the towing experience was not good - the handling was vague and unsettling.

 

I switched to bigger and stiffer LT275/70R18 mud tires (33” diameter). By studying the LT-Metric chart, to match that same target load capability they need to be at 46 psi. These tires are tough and offroad capable, and the extra ride stiffness really settled the truck while towing.

 

Remember that these are MAXIMUM tire loads for that air pressure. The tire and truck engineers understand that many owners run them too low on air and they supply a good safety margin to allow for idiots. You don’t need to run them quite so high if your load is below that weight. If you have a HD truck and you are really loaded down with cargo, you can calculate a good “in between “ value rather than just pumping the rears up to 80 psi as indicated on the door sticker. Also, a warm tire is a happy tire. If yours is hot, it’s a big danger signal and you need to increase the pressure for that axle.

 

I have been running 42 psi in my LT tires for a couple of years now. They seem happy but the front ones are wearing faster than the rears and I may bump my pressures front and rear up to 44 psi to see how the truck drives.

 

Lots of people who switched tire types are running lower than recommended pressure because of the door sticker. Not good..

 

Finally, write down the new pressure on your door sticker, so service techs have a small chance of filling them correctly. See picture.

 

More tire tech from Kimberley Caravans .... https://cdn2.hubspot.net/hub/210453/file-20229690-pdf/docs/importance_of_optimum_tyre_pressure_and_temperature.pdf

 

I hope this stuff is helpful. A truck or offroad forum for your particular vehicle is a good resource, to ask what pressure to run for a particular size. There is also a trick you can do with new tires involving chalk on the tread.... I won’t go into that one. Here is info and a placard you can print for your Ollie tires. .... https://olivertraveltrailers.com/topic/how-to-tire-pressure-placard/

 

John Davies

 

Spokane WA

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  • Thanks 2

"Mouse":  2017 Legacy Elite II NARV (Not An RV) Two Beds, Hull Number 218, See my HOW TO threads: https://olivertraveltrailers.com/topic/john-e-davies-how-to-threads-and-tech-articles-links/

Tow Vehicle: 2013 Land Cruiser 200, 33" LT tires, airbags.

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