Hey everyone,

Tomorrow, after 42,000 miles of driving on the Dunflops, I'm having some Goodyear Wrangler Silent Armor tires put on my 4Runner. I'm going with the LT 265/70/17 Load Range C tires. Thanks to the OP who ran the JP magazine article about the 33 different off road tires, as that helped to refine my choice.

There are a lot of differing opinions about what psi to run different tires at in this forum, some of them rather dangerous. This shouldn't be the case as there are standardized tables to determine this information.

I'm posting URLs for three Load Inflation Tables from Goodyear, Bridgestone, and Toyo, they all essentially say the same thing.

In a nutshell: A stock Trail Edition 5th gen 4Runner with a P265/70/17 tire should be run at 32 psi cold. From the tables (with the 10% reduction for a P-rated tire on an SUV), the load capacity for this tire is 2205 lbs.

When you switch to an LT tire, you need to make sure that your psi allows for your load capacity of the new tire to equal or be higher than 2205 lbs. If you stay with a 265 tire, your psi should be at 44. This gives you a load capacity of 2219 lbs (40 psi is 2075 lbs and 45 psi is 2255 lbs). If you run lower than this, you are overworking the tire. If you move up to a 285/70/17 tire, as many here have done, if you run it at 38 psi then your load capacity is 2231 lbs (35 psi is 2105 and 40 psi is 2315 lbs). This holds true even if the tire is a C, D, or E load range.

Here are the tables:

Toyo: Load & Inflation Tables | Toyo Tires

Goodyear: http://www.goodyear.com/truck/pdf/da...dInflation.pdf

Bridgestone: http://www.tiresafety.com/images/Tir...t%20Manual.pdf

Hopefully, this will help to resolve some of the misinformation about tire pressure on this forum.

This matches what my 285/70/17 Duratracs were set at by the local Goodyear shop that specializes in trucks. I once tried running 42 psi based on some posts here and the truck didn't feel properly planted and jostled over every little bump at highway speed. Went back to running 38 psi since then.

I run 50psi in my 285 Duratracs. At 40psi they lose 1-2mpg like clockwork, and feel less stable to me. I drop to 35 if I venture off the beaten path.

Just wanted to note that according to the manual tire load capacity should be greater than 1/2 GAWR (which is 1719.5 lbs for stock SR5), not the equal of stock tire load capacity:

"Maximum load of tire.
Check that the number given by dividing the maximum load by 1.10 of
the replacement tire is greater than 1/2 of the Gross Axle Weight Ratings
(GAWR) of either the front axle or the rear axle, whichever is greater.
For the GAWR, see the Certification Label. "

And 1.10 thing is only applied to passenger tires, for LT you don't have to divide maximum load by 1.10. At 44psi tire tread don't touch the ground for about an inch from the sides which is fine for fuel economy and noise reduction but will decrease tire grip and you'll need to be extra careful on wet pavement or other slippery surfaces.
Just my observations with Duratracs though.

35 psi might be ok for dirt/gravel roads but I think it's way too high for trails. With some of the rocky trails you go on you're running a greater risk of puncturing a tire at higher pressure. I run 12 psi on snow trails and 15 psi otherwise.

Excellent article on airing down:
The Lowdown, Off Road Tire Pressures

Ya I try to air down more out there. Until I have my onboard air I have to run enough for the drive there.

Note the use of “CAPACITY” in these charts. It’s what the tire, not the vehicle is capable of handling. Also remember that the capacity is seldom the working weight which is also very important. Unless you use your vehicle for working carrying a heavy load you will seldom, if ever, reach 2K/tire and you need to take that weight into account.

Over inflating is just as much a problem as under inflating, especially with trail or mud tires. If you over inflate the tire bulges, reducing tread contact with the road that means reduced road traction. Not a good thing during cornering (especially emergency cornering and/or during wet conditions). Add to this that with trail or mud tires road traction is already reduced do to less tread surface from all those larger lug cutouts. You could easily end up with less than 50 the surface contact area compared to a stock tire if you aren’t getting contact on the outer 1.5”-2” of the tire.

Based on these tables in these charts, most of us with upgraded tires should be around 35psi and never above 40. This also agrees with the tire contact pattern I’m seeing with my tires (285/70-17) which are currently at 35 psi. 

So I should be dunning 38psi on the street by these tables. I don't know about others, but my truck does not feel stable at anything below 40psi on my 285 DT. The mileage loss is fine, but stability is pretty important.

I applaud you for this well written post, most people don't realize you don't need to be running 40-55psi on an unloaded vehicle.

Right, but this is a warning to make sure that you choose a tire whose capability is greater than 1/2 of the GAWR. It says nothing about tire pressure. The stock Dunlops have a maximum load of 2535, The GY S/As have one of 2470.

At 35 psi on a 285, you aren't that far off of what the stock P265 is at 32 psi. In fact, you are only 100 pounds under the load limit of 2205. However, if you are running an LT265 at 35 psi, then you are at 1890 lbs, over 300 lbs under the load limit for stock.

My point in posting this was to make sure that people with LT265s look at the charts so that they can be aware of how underinflated their tires are if they are running in the 30s for psi. And an underinflated tire is deadly due to overheating.

rebel... I wasn't telling everyone to air their tires up to 40-55 psi. 44 psi for a 265 is what Goodyear told Seattle Mike (may he rest in peace) to run on his Duratracs, which is what the charts say.

LT tire load cababilty depends on tire pressure as you can see in load tables. If you look in the Goodyear load table for instance it says that at 65mph stock size C load tire has capability 2740 lbs, at 35psi - 1890 lbs. To me it looks like at 35 psi LT tires are still cabable to support just a little bit more weight then axles are desinged to support. To be on safe side I keep my tires in the range 36-38 psi, but of course I'm not an expert, just trying to think logically : )

Oh and by the way I asked Toyota about tire pressure and they said that it's more a guideline and if LT tires are used then tire manufacturer should provide correct tire inflation numbers. And Goodyear just sent me a tire pressure table with my tires highlighted saying that correct pressure depends on the vehicle weight.

Correct tire pressure can be debated endlessly without establishing a reference point -- and there are lot of them.

From the reference point of matching the load reserve established by the stock spec, the answer is simple -- in this case 44psi. If any reference point can be called official, this is it.

There is a lot of load reserve built into the stock spec, so it's understandable if not everyone feels it's important to match that level, but note the word "reserve". Tires change over time and they don't get stronger. They hold grudges. Allow them to run too hot just once, and the damage is permanent. Load Reserve is a good thing. http://www.toyota-4runner.org/genera...-pressure.html

I don't follow it to the letter myself, but I proceed with eyes wide open. For anyone who has no desire to get into the finer points of tires and pressure, adding 10-15lbs to a same sized LT makes for a good rule.

And there are experts out there.

Ask a tire engineer

Alright, that would imply about 41psi for 265/70/17 which is the pressure where load capacity matches 285/70/17.

Anyone who actually places a high priority on wet and dry cornering and stopping ability should give a lot of thought to whether they really need to switch from P to LT in the first place.

Link please