Tire air pressure

[tedujam]

I'm running BFG KO2 LT285 70R17 E's. I don't have access to a scale, but I'm guessing total weight on my E350 UJOR 4x4 with CCV top and custom interior is about 10k lbs.

Does 55F/60R sound about right? That seems to be the consensus on the threads I've found.

[vlagat]

Seems too low IMO. Check the minimum air pressure for the maximum tire load on the sidewall; multiply by 4 that to show the maximum load supportable by the tires. Then find the proportion that represents of your estimate of 10k. Say that proportion is 0.8 or 80%. Multiply that 80% of the tire placard minimum.

Now comes the tricky bit. Obviously the front axle is loaded way less than the rear. On mine its 62% rear and 38% front. Now you can do the same exercise separately for the front and rear axles except that you will work of the max tire load being doubled rather than X4.

I use 17.5" OTR Michelins because none of the 16 or 17 inch tires are robust enough for a 10k SMB and a 10k trailer and I still have a front rear split of 75/85.

[tedujam]

Originally Posted by vlagat

Seems too low IMO. Check the minimum air pressure for the maximum tire load on the sidewall; multiply by 4 that to show the maximum load supportable by the tires. Then find the proportion that represents of your estimate of 10k. Say that proportion is 0.8 or 80%. Multiply that 80% of the tire placard minimum.

Now comes the tricky bit. Obviously the front axle is loaded way less than the rear. On mine its 62% rear and 38% front. Now you can do the same exercise separately for the front and rear axles except that you will work of the max tire load being doubled rather than X4.

I use 17.5" OTR Michelins because none of the 16 or 17 inch tires are robust enough for a 10k SMB and a 10k trailer and I still have a front rear split of 75/85.

Thanks. Max PSI for these tires is 80, but I don't want to be anywhere near that. I'm really just trying to get some input from the guys around here with E350 SMBs and KO2s.

[Twoxentrix]

if a Load Range E tire is only inflated to 50 psi (with max pressure being 65), it will likely be able to safely carry the same load as a Load Range C tire at the same pressure. Less pressure will decrease the amount of weight the tire can safely carry... and generate more heat going down the road.
The K02's you have should be rated around 3,100lbs, and I suspect an EB like yours when fully loaded will probably be closer to 10,500lbs. With the rear carrying more weight than the front, I'd wager the rear being about 5,800+lbs (that's about 2,900lbs per tire). So you're already close to max at 80psi. Lowering too much may be a gamble.

[vlagat]

Originally Posted by tedujam

Thanks. Max PSI for these tires is 80, but I don't want to be anywhere near that. I'm really just trying to get some input from the guys around here with E350 SMBs and KO2s.

As others have written, without an axle weight, my guess is you are underinflated for that tire at anything less than 80 psi. You may be ok at 78 and with low gas/water/tanks and solo, even 75. I know I was with same SMB.

[doublevan2]

I'm agreeing with Twoxentrix on this tire thing. My 2002 E350 EB comes in at like 10,800 - 11,500 when loaded for bear -- maybe even 11,000 with all my crap on board and a second passenger. I run the rears at 70 and the fronts as low as 54 - depends on how crappy the roads/pavement is - lower off road on trails -

[86Scotty]

Originally Posted by tedujam

I'm running BFG KO2 LT285 70R17 E's. I don't have access to a scale, but I'm guessing total weight on my E350 UJOR 4x4 with CCV top and custom interior is about 10k lbs.

Does 55F/60R sound about right? That seems to be the consensus on the threads I've found.

Yes, it is about right. You are not underinflating an E rated tire at this pressure.

You are properly adjusted for load, and ride.

Enjoy the (comfortable) ride!

[tedujam]

Originally Posted by 86Scotty

Yes, it is about right. You are not underinflating an E rated tire at this pressure.

You are properly adjusted for load, and ride.

Enjoy the (comfortable) ride!

Great, thanks for the input!

[The_Vermonster]

Like others have said, you really need some axle weights to know for sure. I can give my Van's specs as an example. On a recent trip we weighed in just shy of 8k, about 3700 front and 4200 rear.


Our tires have a max load of 3200 at 80psi. So 3200/80 gives you 40lbs per psi.

So the front tires (3700/2) carrying about 1900lbs each get 47.5 psi.

The rear tires (4200/2) carrying about 2100 each get 52.5 psi

I do prefer to add a little safety factor so I ran 55f 60r. I think the higher psi also helps on the highway which was predominately what we were driving.

So my gut says you're a little low at 55f 60r if you really are around 10-11k loaded.

You can also see if you can get a hold of a tire inflation chart like this
https://www.toyotires.com/media/pxcj...s_20200723.pdf

One big difference compared to the math I did is that the load and PSI relationship isn't linear. For example, the difference between 80 and 70psi is 195lbs of load, whereas the difference between 50 and 40psi is 400lbs. Also, 40psi is about 400lbs higher rated than half of the 80psi rating. So according to a chart like that, you might be closer than I give credit.

[Twoxentrix]

Originally Posted by The_Vermonster

Our tires have a max load of 3200 at 80psi. So 3200/80 gives you 40lbs per psi.
So the front tires (3700/2) carrying about 1900lbs each get 47.5 psi.

The rear tires (4200/2) carrying about 2100 each get 52.5 psi

I do prefer to add a little safety factor so I ran 55f 60r. I think the higher psi also helps on the highway which was predominantly what we were driving.

tedujam = Applying the same formula (since you both have basically the same load capacity at 3,100lbs and 80psi max) your figures with a 10,000lb Van (Guessing 4,500front / 5500rear) would be more like this:

Front tires (4500/2) carrying about 2250lbs each get 57.25 (I'd go 60psi)
Rear tires (5500/2) carrying about 2750lbs each get 68.75 (I'd go to 70psi)

If you're currently running 60psi on your rear then you're only inflated for a max load of 2,400lbs per tire (4,800lbs total) - your weight is well over that.
Not trying to alarm you, just becomes a safety concern.