2nd Gen Non-Engine/Transmission Tire pressures

[RRuggles]

Which do I believe or follow? Owners manual says 55# front. 70# rear. Tire itself says 80# max. In the past I've run 70# front, 80 rear with TC on board. Thanks.

 

[GaryOwen]

I'd go with the 70 front/ 80 rear as you have done in the past when loaded. Would use 55/70 when unloaded for a smoother ride. Tire dealers will usually recommend the higher inflation pressures.



Gary

 

[WestTN]

I never use less than 60 around, and my first set lasted 97,000 miles. But, if I were running less, I would have at least 10 psi more in the front. It's much heavier than the rear when empty.

 

[Joe Mc]

The pressures that will work best for you are based on tire size, ride quality desired, your personal preference and load being hauled.



Myself - with the previous 265/75/16 Load Range "E" and no load other than my arse, I used 65 F / 55 R. The ride was not too jarring, control fine, no hip hopping over exp joints in the road and the front never looked bulged due to weight of the CTD.



With my present tires 305/70/16 LR/"E" Nitto Terra Graplers I use 55 F / 45 R, once again no load. The ride is smooth, quiet, no sway or wander and the exp joints appear to not even be there. I have 25k+ miles on these tires and they are wearing perfect. If I load up the back with camping gear I increase the rear to 55. I've had 700 pavers on the back and loaded the rear tires to 65 leaving the front alone and all was well, ride not too soft and handled well for having almost 3000# in the bed. MHO

 

[JFaughn]

Your baseline is the tires , the label on truck can not relate to any tire you put on the truck , only what they put on .

 

[PC12Driver]

Pounds per square inch

The sticker on the door jamb factors in the average contact patch for the specified sized tire and the amount of weight that it carries. The number of pounds per square inch.



Some rough numbers (going quickly by memory):



My front tires on my dually specify 65 psi. Take the max weight is 4500 lbs on the front axle, divide that by two (2250 per tire), and divide that by the rated psi (65), you'll end up with 35 square inches. About the size of the contact patch.



Let's do the same for the rears. 7000 lbs divided by four (1750 lbs per tire), then divided by the specified psi (50) and you'll end up with about 35 square inches. Kinda takes the magic out, doesn't it?



Ever notice that larger (wider) tires specify lower pressures for the same weight carrying capacity? More contact patch X lower psi = same weight rating as smaller tires. Simple math.



On my truck that rarely runs at maximum load on the rear axle I've found that 30 psi in the rears works good for even tire wear. I honestly haven't done the math yet, but let's see what happens: 4000 lbs on the rear axle (guesstimated, since I"m probably 3400 lbs under gross weight), divided by 4 (tires) works out to be about 1000 lbs per tire. Divide that by 30 psi and I get 33ish square inches for my contact patch.



So it looks like 235/85-16 tires are designed to have a contact patch somewhere around 35 square inches. Of course, there will be variances between tire brands and models, but I bet the results will be close when you find a pressure that results in a nice even tire wear pattern (not flat across).



The maximum inflation number on the side of a tire is just that; a maximum. Sure, the tire will hold the pressures, but you'll have a smaller contact patch resulting in less traction, poor tire wear, and of course a ride that's more harsh.



Long story short: the numbers on the doorjamb are correct for your truck at its max weight.

 

[LandShark]

Here's some data direct from Chrysler Pub. # 81-316-9503. This was one of the documents that came with my truck.



All data is for LT245/75R16 E tires that came on the truck.



Light Load: 55psi front, 40psi rear (Club Cab).

Full load: 65psi front, 80psi rear.



The document also has a load chart per tire at various psi's.



PSI Load

35 1700

40 1865

45 2030

50 2205

55 2335

60 2480

65 2623

70 2765

75 2900

80 3042

 

[Grizzly]

Which do I believe or follow? Owners manual says 55# front. 70# rear. Tire itself says 80# max. In the past I've run 70# front, 80 rear with TC on board. Thanks.

I'd use a little more then the owners manual recommends, but don't exceed what the tire manufacture recommends. Adjust the rear for the load you are hauling.

 

[DonTX]

I am following this thread with amazement. I don't want to step on anyones toes, but what is wrong with doing what the tire manufacturer says? All you have to do is go to the brand website, and you have THEIR pressures for each load. Scales are free or cheap, why play with magic potions and such when the information is available for you. Nowhere is there a provision for running the pressure that gives the softest ride or such, you run the correct pressures for the load on the tire, simple as that, it is the reason the tables are made available.

 

[JFaughn]

It has to be written to be believed , rather than figured out I guess .
As sound as you go away from what the dealer knows [ tires truck came with ] they no longer know .
Kinda like the eng. trans. relationship , detune the Cummings , because they did not want to put a trans they didn't have behind it , there recommendations are for there protection [ lawyers ] not ours .

 

[AKaiser]

Bingo: Match TP to LOAD based on tire design specifications. That's a tire manufacturer's call.

 

[DonTX]

What I find noteworthy, is that different makes of tires require different pressures for the same load and tire size even!

Bingo: Match TP to LOAD based on tire design specifications. That's a tire manufacturer's call.

 

[Grizzly]

What I find noteworthy, is that different makes of tires require different pressures for the same load and tire size even!

That's why you have to go by the owners manual, tire pressure guide, what's printed on the tire sidewall and common horse sense.

 

[AKaiser]

What I find noteworthy, is that different makes of tires require different pressures for the same load and tire size even!

It's a function of tire construction.

 

[fest3er]

As a general rule of thumb, I believe you can use the max pressure and the max load for each tire to compute the pressure you need.

Take the Goodrear 245/75/16 OEM tires on my '98. The max tire pressure is 80 PSI and the max load is around 3000#, it means that at 80 PSI, each tire can carry 3000# of load. My 2500 could carry 6000# of weight on both rear tires, or 12000# weight total, if I could load up the front end. (This exercise completely ignore axle and spring capacities. )

Unladen, my truck weighs about 6000#. Dodge recommends 55 PSI front, 40 PSI rear, unladen. That makes the weight distribution 58%/42% front/rear, or 3500#/2500#; this works out to 1700#/1250# front/rear per tire. If I then put 3000# of horse dung in the bed, this changes the distribution to about (guessing) 3700#/5050# front/rear.

A simplified (reduced) formula might be:
unladen pressure + (max - unladen pressure)
* (current tire load - unladen tire load) / (max tire load - unladen tire load)

So, if 40 PSI is good for 1250#, and 80 PSI is good for 3000#, then the proper inflation for the rear tires whilst carrying a 5050# axle load should be:
40 + (80-40)/(3000-1250) = (x-40)/(2525-1350), or about 67 PSI.

Now, my 19. 5" Hankooks are good for 3640# at 95 PSI (3415# dually). With my 6. 5" wheel width, the best tire pressure seems to be about 75/65 PSI front/rear, unladen. The same load of horse dung would require rear tire pressures of:
(95-65)/(3640-1250) = (x-65)/(2525-1250), or about 81 PSI.

This calculation should put you in the ballpark for correct tire pressure whether loaded or unloaded.

An alternate method: if your current tire pressure gives you good wear and performance and proper tread contact, measure the 'radial bulge' at the bottom of the tire. Then put a load on the truck and inflate the tires until the 'radial bulge' is the same. You'll then be pert near the correct tire pressure for the load you are carrying. Alternately, you could measure center-of-axle to ground, then inflate until you reach that height again. Come to think of it, this latter measurement method might just be more accurate, because proper tread contact happens at a certain loaded tire radius, regardless of load. If you can't reach the desired loaded radius without exceeding the tire's max pressure, you're probably carrying too much weight. In this case, your rear leaf springs looking like upside-down 'U's would be a strong hint as well. (With 4000# of horse dung in the bed once, I wasn't yet near the suspension stops, but the springs *were* somewhat inverted ; and I was still well within the wheel, tire and axle limits. )

N

 

[NoSeeUm]

I haul allot of weight at times, most of it on the rear axle. I have a slide in camper and pull a boat. My rear axle weighed in one time at just over 6700 lbms when it was all loaded up for a short trip. At the time I was running 265 tires rated at 3415 lbms @ 80 psi.



All the blood drained from my brain when I looked at my loving wife in the seat next to me.....



After that weigh in, I did quite a bit of research on tire ratings. The ratings are base upon weight the tire is supporting as well as speed. These numbers are based upon the heat generation and heat rejection capability of the tire. Basically, the tire gets hotter under more load and more speed. Providing the tire can handle the weight upon it, it is the heat that most often causes failure.



The tire ratings are a black and white limit, but in reality it is not so simple if you consider the actual capability of the tire vice the tire rating. A tire rated at 3415 lbms @ 80 psi cold for a light duty truck is a good example of a rating. But there also is a speed rating, which is not really given all that much consideration because the vehicle is just not often driven faster than that common speed rating for a LT class of tire. If the same tire never went more than 30 mph then the weight rating "as an example only" might be 4800 lbms @ 80 psi. Consequently, "as an example only" if the same tire had to travel 95 mph the weight rating might be 2300 lbms @ 80 psi.



OK, so all that pre-amble above said one thing to me. It said that I should drive slower as my tires were getting very close to rated capacity or that I should get some bigger tires. AND that my tires should NEVER be marginal in condition. I bought bigger tires. Now I run 285's rated at 3720 lbms @ 80 psi. So far so good.



Better believe I always inflate to 80 psi in the rear and check often for temperature. I run 65 psi in the fronts. When I am empty I run 55 psi front 40 psi rear. I am still checking often for even wear as these tires are still fairly new.



Jim