Tire pressure dilemma - can you help?

[mbradley]

Not knowing what air pressure to run in my new 265/70/17 BFGoodrich TA KO Light Truck tires, the service manager at my local Toyota dealership was kind enough to call Toyota technical support. They stated that the tire pressure listed on the door frame (32 psi. front and back for a 2004 Limited) should be followed regardless of what type of tire is put on the truck.

Since this was contrary to what I had been previously told by BFGoodrich tech support (see my recent post, "BFG TA KO Air Pressure Advice"), I called BFG again. They said Toyota’s advice was not only wrong but could prove dangerous, since I had switched from a stock passenger tire to one in the light truck category. But they refused to give me a recommended tire pressure because BFG TA KO tires in my size are “not recommended” (ie, not on their chart) for a 2004 4Runner.

A professional journalist who makes a career of writing about 4-wheeling, reports to have run 15” BFG TA KOs on his Ford Explorer (you’ll have to ask him why he chose an Explorer). He swears that experience proved the best day to day street pressure for his rig - a few hundred pounds lighter than a 4Runner - was 26psi.

26 to 46psi from the experts - that’s quite a range to choose from! And my owner’s manual clearly warns in bold print that one is risking life, limb and possible destruction of the planet by running a tire pressure that is either too high or too low.

Opinions anyone?

 

[dcampen]

The BF Goodrich data sheet for these tires (C load rated I presume) gives a Max Pressure for these tires of 50 psi.
http://www.bfgoodrichtires.com/assets/pdf/all_terrain_ta_kd.pdf

So, you should be able to safely run these tires at 50 psi. Of course the pressure in the tire will increase as it heats up while you are driving it so if you are checking the pressure in the morning you would want to see a lower value - say 40-45 psi.

As for how low you can go, these tires are rated for 2470 lbs (each) at 50 psi and a 2004 V8, 4WD, Limited has a Gross Vehicle Weight rating of about 5.700 lbs. So, for a loaded vehicle you would want a minimum tire pressure of:
(5700/(4X2470))X50 =
0.577X50 = 29 psi
This is for highway driving. If you were airing down for sand you would of course go much lower.

http://www.4x4now.com/sfjun96.htm

For an unloaded vehicle the minimum highway tire pressure would be a little less. Using the curb weight of 4,500 lbs we get:
(4500/(4X2470))X50=
0.455X50=23 psi

So 23 psi is the absolute minimum pressure for highway driving. I wouldn't feel safe on the highway with the tires at such a low pressure. I would feel much safer with these tires inflated to a cold pressure of about 30 psi.

Caveat: All of the above is just my own wild speculation. Use your own judgement. Don't rely on my opinions.

 

[bulldog-yota]

I would just stick to the 32 psi, maybe a bit more if you like for safety.

You should be OK as the 4Runner is not a truck which carries heavy loads so I dont see the need for high tire pressures.

 

[86runner-sas]

i run my 31x10.50r15 bfg muds on my runner. i run them at 32 psi 15 psi when offroading

 

[mike_d]

to get a good starting place for the proper tire pressure all you need is a little simple math.

first, toyota is wrong, the tire pressure listed is for the particular size of tire that came stock. i don't know what comes stock on the new 4runners, but for the sake of an example lets just say they're 255's.

remember that the product of the contact patch and the tire pressure is equal to the weight supported by the tire (to a first order approximation). so if you go from a 255 wide tire to a 265, you've increased your contact patch by 4%, so you should lower your tire pressure by about 4% from stock.

if the door sticker gives different front and rear pressures, adjust both accordingly, usually the different front/rear pressures are to compensate for not exactly 50/50 weight distribution.