E-350 4x4 BFG Tire Pressure education

[surfgeek33]

We have BFG's 315's mounted on a 17x8.5 wheels. The sticker SMB west placed on our driver side door says:

Front: 45 PSI Max Cold
Rear: 50 PSI Max Cold

Are these the tire pressures I'm suppose to be running? The side walls on our BFG's says 50PSI (max cold). I've been checking our tire pressure the old way with a pressure gauge, since I don't think our Hella TPMS is reading the PSI accurately. Thanks for the help.

-Ray

[Greg In Austin]

The key is the sidewall max cold pressure.

Regardless what any sticker says, you should not go above the max cold pressure on the tire sidewall.

The pressure you use should allign with the load of the vehicle. ...AND the load range of the tire should be able to carry the weight of the vehicle.

[Ford_6L_E350]

Those tire pressures are 'MAX COLD'.

At those pressures, the tires support 6390# rear and 5750# front; or about 12,000# total. Not knowing the weight of your van, that sounds like nice, safe pressures to run.

Alternatively, you could weigh the van axles to calculate a more accurate tire pressure.

The tires are rated at 3195#@50psi. Or, 63.9#/psi. If your front axle wieght is 5000#, you could do the math:

5000# divided by two tires = 2500#

2500# divided by 63.9#/psi = 39.1 psi. Then add 15% for safety and you have 45psi. I added 15% for safety. You may choose more or less of a safety margin. Lower pressures may ride softer. Lower pressures have more rolling resistance and may lower fuel mileage. Higher pressures will ride harsher and may handle better on the pavement. Higher pressures reduce tire temperatures and are a good idea at higher speeds.

Or you could just stick with the numbers SMB put on your van.

Mike

[surfgeek33]

Thank you Mike and Greg for that awesome explanation!

Cheers,
-Ray

[Steve_382]

Originally Posted by Ford_6L_E350

Those tire pressures are 'MAX COLD'.

At those pressures, the tires support 6390# rear and 5750# front; or about 12,000# total. Not knowing the weight of your van, that sounds like nice, safe pressures to run.

Alternatively, you could weigh the van axles to calculate a more accurate tire pressure.

The tires are rated at 3195#@50psi. Or, 63.9#/psi. If your front axle wieght is 5000#, you could do the math:

5000# divided by two tires = 2500#

2500# divided by 63.9#/psi = 39.1 psi. Then add 15% for safety and you have 45psi. I added 15% for safety. You may choose more or less of a safety margin. Lower pressures may ride softer. Lower pressures have more rolling resistance and may lower fuel mileage. Higher pressures will ride harsher and may handle better on the pavement. Higher pressures reduce tire temperatures and are a good idea at higher speeds.

Or you could just stick with the numbers SMB put on your van.

Mike

Mine are rated at 3195 @ 80 PSI, not 50 PSI. If you do the calcs, that would give you 62 PSI plus your 15%, or 71 PSI.

Was that a typo, or are yours really rated at 50 PSI?

Steve

[Greg In Austin]

Different tires with different widths, hieights, wheel sizes, etc, ... will be rated for different weights and at different pressures.

Tires of the same size and load rating category will be rated to carry different amounts or weight.

Our max cold pressure is 65psi, but they are also rated for 3860 lbs each.

the math ends up being the same:

total carrying weight / max cold pressure = lbs/psi

in our case :

3860lbs/65psi = 59lbs/psi

....and they key continues to be...do not exceed the max cold pressure on the sidewall....

[Steve_382]

Thanks,

I also found this table for RV tires that had a size similar to what some of us run on SMBs. Interesting, that at 50% of max tire pressure, they say the tire is good for 61% of max load. I guess they are taking into consideration the structural strength of the tire also.

Steve

From Michelin info for RVs,- LT245/75R16 LRE — XPS RIB® - This tire is rated at 3042 lbs at 80 PSI.
PSI Load in lbs
35 1700
40 1865
45 2030
50 2205
55 2335
60 2480
65 2625
70 2765
75 2900
80 3042

[Ford_6L_E350]

Originally Posted by Steve_382

Mine are rated at 3195 @ 80 PSI, not 50 PSI. If you do the calcs, that would give you 62 PSI plus your 15%, or 71 PSI.

Was that a typo, or are yours really rated at 50 PSI?

Steve

According to 'tirerack.com' BFG All-Terrain T/A KO 315/70-17's are rated at 3195@50 psi. I can't really vouch for the accuracy of that site.

My tires are a different size.

Mike

[Ford_6L_E350]

Originally Posted by Steve_382

Thanks,

I also found this table for RV tires that had a size similar to what some of us run on SMBs. Interesting, that at 50% of max tire pressure, they say the tire is good for 61% of max load. I guess they are taking into consideration the structural strength of the tire also.

Steve

From Michelin info for RVs,- LT245/75R16 LRE — XPS RIB® - This tire is rated at 3042 lbs at 80 PSI.
PSI Load in lbs
35 1700
40 1865
45 2030
50 2205
55 2335
60 2480
65 2625
70 2765
75 2900
80 3042

I've looked for load tables like that for other tires, and never found them. Where did you find that one?

We all know the tire will support some weight with no air. No knowing what that value is, I just derate linearly. That way, the tire won't be overloaded.

Mike

[Steve_382]

This guy did a little research and has a couple of tables with inflation info.


http://www.dmbruss.com/zFullTimeLifeSty ... lation.htm

This is a link to the Michelin RV info. I also couldn't find any info from BF Goodrich.


http://www.dmbruss.com/images/FullTimin ... ochure.pdf