E-350 4x4 BFG Tire Pressure education

[Gene Atl.Beach]

I have just been reading everything I can find since, (under gallery-Blow out picture), on the BFG 285-75-16's..........Seems to me, "D" rated compared to "E" rated is the difference. I have the BFG 285-75-16...D rated and last month had a BIG BUBBLE on the inside right rear. Person behind me saw the bubble and told me....then I see the picture of the tire coming apart....I will be getting all new BFG 285-75-16's "E" rated on Monday. I was going to the smokies and just happen to run across the thread. ...also I run 50 psi in front, 60 psi in rear. I will up the psi on the E rated tire. With the D rated, I get 14 mpg, pulling my 6x12 dual wheel trailer. I will updated this later after my trip..Gene

[Gene Atl.Beach]

I put the new tires on... took the trip to the western end of nc mts. and back to the coast of nc. Got 4 BFG 285's 75 16's "E" rated. Ran 60 psi in front, 80 psi in rear. I did not pull my 6x12 dual wheel trailer, but did have carrier on top. I got 16 mpg.....two more mpg than with the "D" rated tires. Just a better feel with these tires. Also I had a full load of just stuff and water tank full. I am pleased with the tires, feel better about safety. If you didn't see the picture I wrote about in the former post, you need to look it up and keep in mind..... that blow out happen at hwy speeds!!!!!

[ANZAC]

Random old thread bump.

I did some research, and it looks to me like the only things that really matter are:
- the vehicle manufacturer's load carrying capacity for each axle
- which can be derived per tire by looking up the RTA (rim and tire association) load inflation tables for the OE tire size and door placard tire pressure
- the tires that you are running (whether fitted by SMB, Quigley or other -- one assumes Quigley and SMB follow the same process for their stickers, which my van does not have)

On my 2002 V10, the Ford tire placard reads:
60 PSI cold front
80 PSI cold rear
and the OE tire size is 245/75 16.

Looking that tire size up in the Toyo copy of the RTA inflation tables gives (per tire):
2480 lb front load carrying capacity
3042 lb rear load carrying capacity

With a new tire size of 285/75 16, the tire pressures that correspond to >= those load carrying capacities are:
45 PSI cold front (2540lb load carrying capacity)
60 PSI cold rear (3110lb load carrying capacity)

Interestingly, the load inflation tables only stipulate D rated tires for the rear at those loads. There's no harm in having E rated tires, of course, and stronger sidewalls are better at resisting off road impacts and rocks.

Toyo load inflation tables: (see p7 for step by step process)
http://cache.toyotires.com/sites/defaul ... nTable.pdf

These should be the same for any brand of tire, because the load carrying capacity relates to the size of tire and amount of air pressure in it. (obviously the tire has to be rated to be used up to a given capacity)

I don't think scaling tire pressures using "lb per PSI" math is a good idea. The tire tables represent some amount of research and calculation.

I think the SMB build out (which adds weight) doesn't change anything. It is like a cargo van carrying a bunch of bricks or tools in the back, it is just extra weight, providing you don't go over the total load rating for the van - which Ford specifies.

I'm currently running 55/60 pressure, so will try 45/60 and see how it feels. But based on what I have looked at above, they look like the correct pressures for 285/75 16.

Incidentally, if you have 285/70 17, the numbers come out at:
45 PSI front (2510lb LCC)
60 PSI rear (3070lb LCC, D rated required)

The only unknown I can see is, SMB or Quigley changes out the front axle, so not sure if that changes anything.

And of course, I'd like someone to prove my approach as wrong.

Disclaimer: if I'm wrong and you kill yourself because of what I posted above, it is your fault for not doing your own research or trusting the tire placards.

[jage]

And of course, I'd like someone to prove my approach as wrong.

Well, you used the OE plaque and non-OE tires in the same size to determine the theoretical front and rear weight of the van... I'd say that's an erroneous way to start - even if the answer is correct in the end, you don't get points if you're method is incorrect.

[ANZAC]

I used the OE tire size 245/75 16 to look up the OEM load capacity at the OEM pressure 60/80.

The brand of tire males no difference. The Toyo tables are from the Tire and Rim Assoc.

Read page 7 and get back to me.

Also it is not "weight", the critical piece is load carrying capacity.

[ANZAC]

More info. Other than the maximum load carrying capacity, which I am sure relates to tire construction, the relationship between PSI and load carrying capacity is just math, but it isn't "PSI per LB". It looks to me like it relates to size of contact patch x PSI.

So for example, if you have a wider tire, you need a lower PSI for the same load carrying capacity. I just ran these numbers over the entire load inflation table, and it comes out very close.

The larger diameter tires have a slightly longer contact patch, which reduces the PSI a bit, and of course at higher PSI you reduce the patch as well, so the numbers are a bit interactive, but width seems to have the largest impact on PSI per lb LCC. I'll spare you the spreadsheet. I used section width and scaled it down a bit for tread width, and basically divided the load by the PSI, and then by the tread width, to get a contact patch length. (ie one side of the load rectangle)

So if you think about it, all of the load is applied to the contact patch, and the air pressure is even inside the contact patch..... each square inch of contact patch can carry up to xx PSI of what you have it aired up to.

Anyway, back to how to use the tables, here are the instructions from page 7, with my examples in italics.

Basic procedures for reading and applying the load inflation tables:
Original Equipment

1. Locate tire information placard to confirm OE tire size and cold inflation pressure. ==> 245/75 16, 60 PSI cold front 80 PSI cold rear

(The tire information placard can be found on the vehicle door edge, door jam, glove-box door, or inside of the trunk lid.)

2. Identify the standard used (TRA for P-metric, LT-metric, and flotation sizes and ETRTO for Eurometric sizes) and refer to the appropriate load inflation table.

3. Find the corresponding load for the OE tire size(s) at the recommended cold inflation pressure.
==> 2480 lb front, 3042 lb rear load carrying capacity

Replacement Tire

4. Use the appropriate load inflation table for the replacement tire size(s).

5. Find the inflation pressure to which the corresponding load is equal to or greater than the OE tire.
==> for 285/75 16
45 PSI cold front (2540lb load carrying capacity)
60 PSI cold rear (3110lb load carrying capacity)

6. Inflate tires to the appropriate inflation pressure.

7. If the replacement tire requires a different inflation pressure than OE, the installer should inform
the owner of the new required inflation pressure and should also place a sticker or decal over the
vehicle tire placard showing the new tire size and recommended inflation pressure for future
reference.

[rionapo]

Not sure whether you want to run the tires higher or lower and not speaking from SMB experience: I originally ran my tires overinflated on a Chevy K2500 heavy with a 3500 lb. Lance cabover -- thought the weight needed more pressure -- and wondered why the tires didn't last. A tire salesman finally clued me in and I dropped them down to recommended levels. Truck handled better, tires lasted longer. And I didn't blowout the sidewalls anymore, either.

[haywoodphotomaccom]

Table I am sure is on here somewhere already but here it is again. My van is 6800 total split almost evenly. Assuming I can go down to 35 PSI or a little less for each tire? Thinking of switching to D rating for softer ride even though mine are only a few weeks old. I only put 500lbs or so in cargo. Thoughts?


Listed below is the load and inflation information for an LT265/75R16:
*
35 PSI - 1910 lbs
40 PSI - 2100 lbs
45 PSI - 2280 lbs
50 PSI - 2470 lbs
55 PSI - 2625 lbs
60 PSI - 2790 lbs
65 PSI - 3000 lbs
70 PSI - 3105 lbs
75 PSI - 3260 lbs
80 PSI - 3415 lbs
*
We appreciate your business and thank you for choosing BFGoodrich.

[ANZAC]

Originally Posted by haywoodphotomaccom

Table I am sure is on here somewhere already but here it is again. My van is 6800 total split almost evenly.

That is the weight. The tables are for load carrying capacity per tire....

Listed below is the load and inflation information for an LT265/75R16:
*
35 PSI - 1910 lbs
40 PSI - 2100 lbs
45 PSI - 2280 lbs
50 PSI - 2470 lbs
55 PSI - 2625 lbs
60 PSI - 2790 lbs
65 PSI - 3000 lbs
70 PSI - 3105 lbs
75 PSI - 3260 lbs
80 PSI - 3415 lbs

From my prior post, OE van tire size carrying capacity is 2480 lb F per tire LCC & 3042 lb R per tire LCC at door placard pressures.

Based on the table you posted for the new tire size, you would want to be at 55 PSI front to give 2625 lb LCC (though you're pretty close to 50 PSI which is 2470lb LCC) and 70 PSI rear giving 3105lb LCC.

The method is to pick the inflation pressure which gives you equal or greater load carrying capacity to the OE size at door placard pressures.

Again this assumes that Quigley/SMB didn't come up with new axle capacities etc. or somehow vary the tire placard.

[haywoodphotomaccom]

Anzac, so why not run around 35 PSI on all tires if they are all roughly taking the same load? 1910 lbs each time at 35-40PSI. 7600 lbs carry capacity and van is 7000ish.