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Discussion Starter #1
This is embarrassing…I thought I was smarter!

I’ve read all the charts I can find…searched the forums…and still can’t figure this out.

GMC ext. cab dually D/A
Empty weight:
4,240 lbs front
3,080 lbs rear
LT215-85/16 load range E tires

For ride comfort primarily and tire longevity secondarily, what are the appropriate inflation pressures for these 6 tires for solo highway speeds up to 80 mph? And, based upon the inflation charts, how does one arrive at those numbers?

Next question:
Same truck loaded with 5er and gear
4,360 lbs front
5,560 lbs rear
For tire longevity and safety, what are the appropriate inflation pressures for these 6 tires for towing at highway speeds up to 65 mph?
<DIV>
</DIV>
 

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Don't know the answer the question but our shop runs a 1 ton dually d***e and I have worked there for way to long. We used to change tires just about every year on this truck till we stuck Michelines on it. This last time it has been 3 years. We just run the pressures listed on the door and it has a flatbed on it. Runs anywhere from empty to fully loaded with a goose neck (old and drags like a sled) with equipment or tractors on it. The Michelines not only lasted longer they rode better. They also are not the popular wide tread tire but are the load range E. They were pricey but more than paid for themselves in the end. Also we religiously rotate every 10k miles.
 

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Here are the "rules" I use.


First, running tires more than 5 psi over OR under the inflation required for a given weight will increase tire wear.


Second, each tire in a dually configuration (rear) will carry less weight at the same inflation than the same tire in a single configuration (front), but I have four tires sharing the load in the rear.


My '03 LS Crew Cab 3500 weighs 4010 front and 3120 rear unloaded. About 60% or the driver and front seat passenger weight goes on the front tires, and about 55% of any rear seat weight goes on the front. I measured the approximate center of the "load" from the front axle and the rear axle to determine this.


Now, for actual pressures in my stock LT215/85R16 tires:


Front singles: 55psi carries 4100 lbs, 60 psi carries 4360, and 65 psi (max for my tires) carries 4670 (FAWR). So I run 65 psi front.


Rear duals: 40 psi carries 5960, 45 psi carries 6500, 50 psi carries 7060, 55 psi carries 7460, 60 psi carries 7940, and 65 psi carries 8600 (RAWR is 8550). As I recall, I should actually be running 35 PSI rear when empty, but since the load changes frequently, I leave them at 45 psi when not towing. This results in a harsher ride than 35 or 40, but I don't have to air up and down several times a day. Also, my experience has taught me to always run about 5 or 10 psi higher than the actual load requires to maximize tire life (at the expense of a soft ride).


Transferring these numbers to your load, I would run the fronts at 65 psi at all times, and the rears at 40 when empty, and maybe 50 or 55 loaded with your 5er and gear.


Hope this helps,


Ralph
 

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Washboy,


Can't tell you from practical experience with a fiver since I don't have one yet.....



I've run a number of "Dooleys" now and generally they run empty or with an ATV in the back or maybe a two horse trailer. Difference here is that most of my trucks except the first 1 ton were gassers which weigh a bunch less on the front.


So for now, I'm running 60 psi on the fronts and 40 psi on the rears which is a rougher ride than the old truck, but this one's got a higher GVW as well. When I get the fiver, I'm going to start with 50 on the rears and 65 on the front and see how it goes. You can usually tell if more air is needed if you are way underinflated by how the truck feels. Tire wear will be higher with the lower pressures as noted above, but that's not too much of a consideration for me as long as the safety aspects are not compromised.


I thing Ralph has given you some good advice.
 

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With just about any tire, the sidewall temp will give you a good indication of under inflation. After running for a while pull over and feel the side wall. Proper inflation for your load and speed will make the sidewall warm. Under inflation will make the sidewall HOT! This is not the most scientific test, but it works.
 

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Discussion Starter #6
<DIV>Ralph,</DIV>
<DIV> </DIV>
<DIV>Thanks so much for the reply and explanation of how to read the tire pressure chart. I think I get it now...not!</DIV>
<DIV> </DIV>
<DIV>Divide axle weight by the number of tires on the axle. Look at the chart for the tire size and weight on each tire...the corresponding psi recommendation is at the top of the chart. Right?</DIV>
<DIV> </DIV>
<DIV>If so, Empty truck is:</DIV>
<DIV>4,240 lbs front divided by 2 = 60 psi</DIV>
<DIV>3,080 lbs rear divided by 4 = so low it's off the chart</DIV>
<DIV> </DIV>
<DIV>Loaded truck is:</DIV>
<DIV>4,360 lbs front divided by 2 = 70 psi</DIV>
<DIV>5,560 lbs rear divided by 4 = 35 to 40 psi</DIV>
<DIV> </DIV>
<DIV>See! I'm still confused! What the heck am I missing?</DIV>
<DIV> </DIV>
<DIV>I've been running 80 psi(!) in all positions when towing at the recommendation of several tire and RV dealers...I know this is not accurate as the tires are wearing poorly and the ride is WAY too harsh.</DIV>
 

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Washboy,


Yes, it looks like you got it. Just be careful that you're reading the right lines in the chart, since the "tires used as singles" and "tires used as duals" are different values.


Also, don't forget to allow for that 60/40 distribution for the front seat. And even though the charts might indicate you need no air in the rear when empty, I don't think I'd ever run them less than 35 psi.


Ralph
 
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