Tire air pressure

[CDACamper]

Many of the replies above are from people running 80 max psi tires. Your 315 tires are likely 65 max psi so you are likely looking at lower psi. I'm running bfg ko2's 305 70 16 max psi 65. Van weight is around 10500 loaded with 6.2k on rear axle and 4.3k on front (4x4, penthouse and eb50). I run 42 psi in front and between 55 and 60 psi in rear depending on gear I'm packing. This is based off or tire pressure charts.

[Twoxentrix]

Originally Posted by CDACamper

I'm running bfg ko2's 305 70 16 max psi 65. Van weight is around 10500 loaded with 6.2k on rear axle and 4.3k on front (4x4, penthouse and eb50). I run 42 psi in front and between 55 and 60 psi in rear depending on gear I'm packing. This is based off or tire pressure charts.

Just going to throw this out there; Tire load ratings and characteristics are published using their recommended 'max' tire pressure (optimum performance for the tire).
At 10,500lbs I would be concerned with the front tire driving characteristics when you're at 42psi - It may be adequate in terms of load carrying ability but at that pressure your sidewalls probably will not provide the strength you envision when confronted by unexpected/unfamiliar turns (quick & abrupt, slalom roadways, and higher speed turns which create more "G" force). The farther you are off the max pressure, the less rigidity the sidewall possesses, and with an off-road style tire (which has considerably higher sidewalls) I'd be concerned if I had to make a abrupt evasive maneuver to avoid something...like debris in the road, or another vehicle... or...
Just considering safety here since you're about 35% under the max air pressure in an arguably heavy van.

[CDACamper]

I have been tempted to run higher psi in the front because it is seems really low particularly compared to the rear which is in the 55 or 60 range. But the rear axle has 2000+ lbs more weight which is a 50% increase over the front so I guess it makes sense. My calculation for front tires based on my weight and tire size suggests 35 psi and I upped it to 42 for an added safety margin. If you are running smaller volume 80 psi tires these pressures are way off. For the OP who is running 35" tires that have a lower max psi of 65?? and higher load rating (assuming E 10 ply) I think the above is in the ballpark.
Sharing the same concern you have, twoxentrix, I have experimented between 35 and 45 psi in front and from a seat of pants feel in cornering there isn't any noticeable difference when pushing it in corners. The sidewall even at 35 psi looks the same as the rear. The only difference i notice is a rougher ride when at 45 versus 35 and 42 seems the sweet spot. I'm curious what size tire/max psi are you running, your van weight and psi front and rear you are running? Always good to see comparisons.

Something I was told by a tire tech a few years ago is to slide a business card between the road and tread on the side of the tire. If it won't slid in its under inflated if it slides in a 1/4 in its good and if you can go more than 1/4 is has too much air. This was what he suggested when figuring how much air to put in oversized 35 inch tires on a lifted 4runner I had. I never tested that but thought it was an interesting concept.

[Twoxentrix]

Originally Posted by CDACamper

I'm curious what size tire/max psi are you running, your van weight and psi front and rear you are running? Always good to see comparisons.

I'm running Toyo M/T's 35x12.5x17 (65psi max). Loaded, I'm at 11,800lbs (5k front & 6.8k rear) and run 59-60psi on the front and 65 rear. Realistically sometime next year I'll probably move to a higher load rating since I'm already challenging the current limits of my M/T's.

[Bellpilot]

I have been running BFG KO2’s 285/75-R17 and stayed away from the 35’s -315’s just so I can run up to 80 psi in them if need be. I usually run 65 psi in the front and 70 psi in the rear because the 3165# capacity shows it starting at 65 psi. My axle weights are 4650 front and 5900 rear. I have been looking for an alternative to the BFG’s in the 35/315 sizes but not a lot of tires allow the 80 psi. My 2010 SMB also has the tire monitor which is a pain and bright as heck if your not maintaining the recommended tire pressures.

[CDACamper]

That's interesting. I went with bigger tires for the opposite reason. I didn't want to have to air up to a much higher air pressure to achieve the same weight load capacity. Easier on my compressor and less likely to have tire damage with lower psi. It's a pretty big difference pushing 55 psi into a tire versus 70 or 80 psi. My larger tires will hold more weight at 65 psi than your size tire at 80 psi so being able to put more air in does not really help and running higher psi on dirt roads makes the tire more susceptible to puncture.

[Bellpilot]

I see your point CDACamper. What are you running for tires? What I have found and maybe I am wrong , to get the 3165 # rating for the Toyo MT 35-12.50 or the LT 315’s you would need to run 60 psi? The BFG’s have a 3165 # at 65 psi so I have what I think is more margin not having to be at 80 psi to get capacity. This is a great discussion!

[CDACamper]

I agree, this is a very good discussion, learning as we go! It’s helpful to see what everyone is doing. It is too bad there is not more definitive information out there with respect to pressure so I’m only going with the pressure charts available online and hope they are accurate.
I am running BFG AT KO 2’s that are 305 70r16 with an E load rating. It appears that the 16 inch rim size makes a sizable difference in weight rating compared to a 17 or 18 inch rim, I guess because of the volume of air in the tire is greater to dissipate heat?? Here is the tire inflation recommendations for your tire size and mine.

https://photos.app.goo.gl/Eoo4WZ7yehvQonfx6

[SheepShagger]

For me it was about ride, tire footprint & air volume. The lower the PSI you can run, the better the ride quality (within reason). I hated running 285’s at ~65 PSI to get the load rating, way nicer ride quality running 315’s at ~45 PSI to get the same load. As an advantage, while the volume inside a 315 tire is more, it actually takes a lot less air to fill then to the needed PSI as can be seen from the chart I created years ago when I made the switch. (97 gallons vs 121 gallons)


I don’t follow the margin comment though. If the tire is rated at 4K and weight on that side of the axle is 3.5k it’s still the same margin assuming you have the correct PSI. Seems the only difference is you can increase the load rating in smaller increments since you have more PSI to play with. This is also very much assuming that the load vs PSI are linear which they aren’t.

[Bellpilot]

Great info SheepShagger. And great catch CDACamper on my wrong weight rating. I placed a 6 where a 9 was supposed to be. # 3195. Again this is great info. because this spring I need a new set of treads. I just need to get my tire pressure monitor recalibrated so I can still safely know if I have a low tire and not have a glaring indicator on the dash all the time.