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Tire pressure in 37" tires

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Old 01-20-2010 | 09:25 PM
chambers1517's Avatar
chambers1517
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Tire pressure in 37" tires

I have a 04 6.0 with 37x13.5 Toyo's on it. Most people run way more air in their tires than needed. I normally let air out until the tire sits flat on the ground. To get a good footprint I had to go to 40psi front and 30 psi rear. This makes sense sinct the front should be heaver than the rear. What I don't understand is why the sticker on the door says 50psi front and 55psi rear. I had a jeep with 36's on it and I ran18psi on it. The tires wore great.
 
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Old 01-20-2010 | 11:12 PM
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CanadianSmokie
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Tire pressure is dependent on a few things. If you want decent fuel mileage, running a lower pressure will kill it. Also running lower pressures will build up more heat in the tire and possibly reduce the tread life. It will also cause the tire to feel squirrley while driving.
With E rated tires, you can almost run no air pressure and still not be on the sidewalls, so running the pressures you are saying to get a good footprint isn't necessarily true. You would get the same footprint if you ran at, say 50-55 psi. I run my 315 E rated MT's at 55psi and find the fuel mileage is decent with good even tread wear. If I wanted better fuel mileage, I could bump it up to 60+ but my ride quality would suffer and I would be prematurely wearing out the center of the tread.

Just my take on it...
 
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Old 01-20-2010 | 11:48 PM
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soutthpaw
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If you have to go that low then you probably need wider rims, a 10+ inch would be optimal... also the specific tire will effect how much it curls up at the corners or how flat it sits on the road at higher pressures
 
  #4  
Old 01-21-2010 | 07:39 AM
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tdappleman
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How are you figuring out your footprint? You can either use the chalk method or a tire pyrometer. The chalk method is fairly easy but typically not very accurate. The pyrometer takes practice to really get accurate readings - especially if you are trying to do all 4 tires at the same time. Rubber isn't a very good conductor of heat so it can be challenging. For the calculated load of the tire the manufacturers provide that and it is at the max tire pressure and is usually labeled as such right on the tire. I wouldn't tow anything with the rears set to 30psi. For comparison, my Ex with 285's are running 55 F and 58 R and the Bronco with 33x12.50's is running 28 F&R unless I'm going play in the sand. You may want to call Toyo and get their recommendation and use that as your starting point.
 
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Old 01-21-2010 | 09:04 AM
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chambers1517
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I agree, I would never tow at 30 psi. I figured my footprint by spraying water on the driveway and driving through. These are the max pressures where the entire tread was visible in the tracks. I also have a dusty road I travel going to work and this is the pressure where no clean tread is left after traveling this road. I had spoken several years ago with a tire manufacturer and learned that he volume of air in a tire holds the vehicle up more than the pressure. They said the correct way to determine correct pressure is to weigh the front and rear and calculate the correct pressure. My current tires are rated 4300 pounds max at 65 psi. So 4300/65=66lbs per psi. If I knew the weight carried by each tire I would divide that by 66 to get the correct pressure or if I knew the weight on the front and rear tires i would use 66*2=132lbs per psi per axle. This is how I figured for my jeep but I have yet to weigh the excursion.
 
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Old 01-21-2010 | 09:26 AM
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bcrewcaptain
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From: Central Va
call toyo and see what they say, WAY too many variables, the way you're doing it just isn't going to work, each tire is going to be different as far as sidewall strength. The Steelteks that came on the truck I could pull the valve stem and still not collapse the sidewall, the MTX's that are on there now squat at about 18PSI when I'm getting ready to hit the sand. I have an offroad rig with 36's I run at 4PSI offroad and 12 on road, you just can't go by what the tire says or the door sticker and they only apply to one scenerio.
 
  #7  
Old 01-22-2010 | 07:15 AM
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I run (normal street driving, low load) my 315/75x16 Toyos on 8inch rims on 48 psi front, 46 psi rear. As recommended by the Arctic Trucks, testing and delivering the customizations done to my rig.
 
  #8  
Old 01-22-2010 | 08:53 AM
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jdadamsjr
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The air pressure for YOUR rig and YOUR tires, etc... will be similar to others, but to ensure it is the proper pressure for YOUR conditions do a search for the chalk test.... (chambers1517's water patch test is a variant that is good too !)

gives you the proper contact patch for your vehicle...

good luck
 
  #9  
Old 01-26-2010 | 06:26 PM
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dzldenny
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We just bought a set of 315/75-16 Pro Comp ATs for our Ex and mounted them on 10 inch rims. Don't even have them on the truck yet. I called Pro Comp to see what they recomended for pressure. He said each vehicle will be different. IE an 8000 truck can run the same tire as a 3000 truck, but will require different pressures. He said to use the chalk test. He said to run the highest pressure possible and still have full contact.
 
  #10  
Old 01-27-2010 | 06:03 AM
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chambers1517
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Yep, that is what they told me. M<ine is 40 front 30 rear. Any more and I lose full contact.
 
  #11  
Old 01-27-2010 | 06:08 AM
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chambers1517
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4.56 gears and spedo

I have 37" tires on my Ex and the speedometer is off about 13%. It is in the shop getting 4.56 gears installed which will bring it back close to factory gearing. My question is will this also bring the speedometer back in line?
 
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