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Tire Pressure Question

M

misweetrevenge

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Just bought a 91 Burb yesterday and was looking at the tires when I got home checking air pressure and such. Noticed the tires are only at about 25 PSI. The sidewall says max cold is 50 PSI. On my cars I usually have it about 5-8 psi lower than the sidewall max. now these tires are 33x12.5x16. tread is 2 ply steel + 2 ply polyester. sidewall is 2 ply polyester. side walls are a little cracked. should i bring it up to about 40 or 45 psi or leave it low so theres no an extreme amount of pressure so the tires don't blow? BTW until i replace the tires it is my around town vehicle. not much freeway (maybe an exit or two)
 
Factory was probably about 30-35. Since the tires are now bigger 25-30 psi is about right.
 
While increased pressure will put more strain on the cracks, they are more likely to give out due to the extreme flexing caused by low pressure.

Air'em up.
 
No, they weren't.

Max PSI is just the max pressure that those tires can sustain while holding up their max load capacity. If that's 3,000lbs each then that'd be 12,000lbs total weight capacity.
 
+1 to AJM. Tire pressure is based on the load the tire sees, which the manufacturer of the tire has no way of knowing. As he said, the sidewall pressure is max, and clearly states that. Not "ideal". That is why each vehicle has/had the specs printed on a sticker in the door jamb.
 
Mine are load range d and are 50 lbs max at 3200 lbs. I run 30 up front and 25 rear as my truck weighs about 5000 lbs. you have to put as much air in as the weight of the truck plus load requires. Max air pressure will balloon and crown the top of the tire and wear out the center faster than the rest of the tire. On the other end of the scale under inflated tires will overheat the tread and plys and also hurt the tire. I have Learned this a lot driving trucks for a living. For every couple of pounds of under inflation the tire gets 10 degrees hotter. This is why you see so many tires Blown out in the summer. You have to find the happy medium with automotive tires.
 
One more for between 25 and 32. The only thing you are doing with putting higher pressure in them is wearing out the centers making the ride worse, and getting a tenth mpg better fuel mileage.

My Jimmy with 35s ( same width) get 32 in the front and 27 in the back when unloaded or not towing, when loaded and or towing the fronts stay the same backs go up to 40 to 45 psi
 
If I'm running on the street, I run mine at 40PSI, and if I have access to a compressor, run them at 15-20 off road. I'm not worried about wearing the centers out since the tires dry rot before the tread wears appreciably. I arbitrarily arrived at 40PSI.

The ride difference between the two pressures is immense, and honestly, I don't know as the ride quality at 40PSI is worth the tradeoff in MPG, whatever microscopic amount it might save. Maybe I should try more just to see if 40 isn't enough to notice MPG improvement. But that is going to depend on the tire construction too.
 
For big tires pretty much all rules are out. For 40" tires you can get away with running like 20 psi and never know the difference...

For towing though and normal sized tires -
Get your truck weighed on a scale. The calculator needs front axle and rear axle weights. Then you'll need the weight rating of your tire and maximum air pressure rating. Then you plug the information into the calculator and figure out when your optimal tire pressure is. For instance a dodge diesel below for running around empty:

Information

Front Axle 4,420
Rear Axle 3,040

GAWR

Front Axle 5,200
Rear Axle 6,084

Tire

Max Tire Weight 3,042
Max Air Pressure 80 PSI
_______________________________________________________

Calculation (Axle Weight / 2) / Tire Max Weight x Tire Max PSI = Inflate Pressure

Front Axle = (4,420 / 2) = 2,210 / 3,042 = 0.72 x 80 = 58 PSI
Rear Axle = (3040 / 2) = 1,520 / 3,042 = 0.49 x 80 = 40 PSI
 
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