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Old 08-04-2015, 01:31 PM   #15
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Slowmover and crispyboy, it's a brand new 2015 truck that we special ordered and have only been driving for two months.

I just took another look at the tire information supplement, which says it is normal for the tire pressure to increase from 2 to 6 psi during operation. On the drive out to our destination, the increases remained within that range (according to the TPMS). On the drive back yesterday (after we'd adjusted tire pressures to the number on the placard), the pressures increased significantly more than that, and that worries me. I do wonder about the accuracy and reliability of the sensors.

I checked the truck tire pressures again this morning-- today two of the tires on opposite corners are spot on and the other two are a little high. On a side note, I have noticed that the Firestone Transforce AT tires they came with receive very mixed reviews.

Thanks for the tip on the Longacre gauge, slowmover. Apparently I wasn't paying much attention when I purchased the TireTek gauge earlier this year-- it's not like I haven't read numerous threads on tires and tire pressures on these forums and I should have noted the limited range on that gauge. I'll put it in our other vehicle and buy one with the proper range for the truck. That will be so much faster than dragging out the compressor to check tire pressures.
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Old 08-04-2015, 01:43 PM   #16
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On the drive back yesterday (after we'd adjusted tire pressures to the number on the placard), the pressures increased significantly more than that, and that worries me.
This is a typical indication the tires were under inflated for the load. Under inflated tires will generate more heat when in service than a properly inflated tire. This greater heat increases tire pressure.

Increasing tire pressure will keep the internal heat from forming. Too much increased tire pressure and the tire to road contact patch will be an issue. It is a Goldilocks thing where just right is the best place to be.

>>>>>>>>>>>>Action
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Old 08-04-2015, 02:10 PM   #17
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Quote:
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This is a typical indication the tires were under inflated for the load. Under inflated tires will generate more heat when in service than a properly inflated tire. This greater heat increases tire pressure.

Increasing tire pressure will keep the internal heat from forming. Too much increased tire pressure and the tire to road contact patch will be an issue. It is a Goldilocks thing where just right is the best place to be.

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Yesterday I mentioned to my husband that we have got to find the chart that indicates what psi for a given load, but am having trouble finding one. I think I've seen links here before but can't locate them. Can anyone help?
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Old 08-04-2015, 02:34 PM   #18
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I believe those charts are tire manufacturer specific and definately tire size specific. ( I might be wrong) I would suggest finding the tire manufacturer website and go searching there.

So tire pressure/load differences on a Ford Focus tire are going to be much different than a Ford F150.

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Old 08-04-2015, 02:41 PM   #19
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I believe those charts are tire manufacturer specific and definately tire size specific. ( I might be wrong) I would suggest finding the tire manufacturer website and go searching there.

So tire pressure/load differences on a Ford Focus tire are going to be much different than a Ford F150.

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Yes, I meant for the specific tire-- but I'm not finding it at the Firestone website... which means it's probably staring me in the face!
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Old 08-04-2015, 02:42 PM   #20
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Oops... never mind. My husband thinks he's found it.
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Old 08-04-2015, 02:52 PM   #21
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Tires will increase pressure with heat. Any kind of heat. If I remember posts from eh tire gurus 10% increase in pressure is normal. What didn't sound normal was you said the pressure was at 63 (not sure if that was indicated on the dash or you measured it), door sticker says 60 and you got an "UNDERINFLATED" reading. Sounds like a computer or TPMS sensor error if it gave an Under warning.
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Old 08-04-2015, 03:05 PM   #22
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http://www.trucktires.com/bridgeston...loadtables.pdf

Firestone is owned by Bridgestone. Follow the link on page 8 or further down may help.

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Old 08-04-2015, 03:40 PM   #23
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With the stock Michelin LT265/70R17E tires on our 2012 Dodge, the front axle is rated 5,500 pounds and the recommended tire pressure is 60 psi and the rear axle is rated 6,010 pounds and the recommended tire pressure is 70 psi. I run the pressures 10 psi higher at each location when towing.

I use a Longacre digiial tire gage to check tire pressure while the trailer is still under the roof in the shade at the storage unit. Cold is a relevant number when the ambiant air temperature is 105+ degrees out.
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Old 08-04-2015, 04:06 PM   #24
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Tires will increase pressure with heat. Any kind of heat. If I remember posts from eh tire gurus 10% increase in pressure is normal. What didn't sound normal was you said the pressure was at 63 (not sure if that was indicated on the dash or you measured it), door sticker says 60 and you got an "UNDERINFLATED" reading. Sounds like a computer or TPMS sensor error if it gave an Under warning.
I know-- it's bizarre. The tire pressure readings were from the dash. The lower of the two readings on the front tires before we left town that morning was 59 (recommended psi is 60 on the front tires according to the label on the door). When we got out to check the tires manually after the alarm went off, we realized the TireTek gauge I'd purchased didn't go above 60. My husband purchased another tire gauge inside a truck stop but it was junk and he couldn't get a consistent reading from it (I've just ordered a Longacre gauge that will cover the range we need, and it will be here Thursday). We did find that the gauge on our air compressor was always about two psi higher than the readings from the dash.

The next morning, at an altitude 2000 feet higher than home (with much cooler temperatures), the tires were all low and we added air. Before we left for home the next day we checked the tires again. Overall, the two rear tires each got an additional 5 psi to reach 80 psi. And on the drive home, the TPMS said that the psi in those rear tires increased by 13-16 psi, which I
found alarming. The tire supplement says an increase of 2-6 psi is normal, but the manual is generic to most automotive tires and says nothing about towing.

With the new gauge in hand, I will see how it relates to what the truck's TPMS is telling us. Then I will do my best to stop obsessing over the TPMS (I'd never obsessed over it before this incident and there's always the chance that it is faulty), and start obsessing over checking the tires manually every morning before we start, and every time that we stop during the day, and noting it, along with tire temperatures, because gosh I do love gathering data and observing trends. No... really, I do.
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Old 08-04-2015, 04:50 PM   #25
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The important measurement is the one in teh morning before you take off. Measuring after you have left is OK and I would not adjust pressure on a tire that has been on the road. Not until the next day.

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Old 08-04-2015, 05:47 PM   #26
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And don't be surprised if the TPMS in the truck isn't accurate to the tire gauges. Mine is off 3-5#. Dealer says that is normal margin of error. Pressures checked with two digital gauges, a Longacre and calibrated gauges at Discount Tire. All of those are in agreement. TPMS system is the only voting off of the island. Need to look and see if GMC has a calibration method.
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Old 08-04-2015, 06:27 PM   #27
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http://www.trucktires.com/bridgeston...loadtables.pdf

Firestone is owned by Bridgestone. Follow the link on page 8 or further down may help.

>>>>>>>>Action
Thanks for the link.

Okay, we have the LT275/70R18E tires. Max weight on the front axle is 6000 pounds, recommended max pressure of 60 psi. Max weight on the rear axle is 6500 pounds, recommended pressure of 80 psi.

When we took the rig across the CAT scales our first weekend out, the weight on the truck's front axle was 4600 pounds, the rear axle was 5060 pounds, and the trailer's axles weighed 5760 pounds. I doubt that changed much this time around. Is there anything wrong with these numbers?

So if I look at that chart, it says that the maximum load on one of these tires at a cold inflation pressure of 60 is 3020, and we had about 2300 pounds on each of those front tires (assuming the truck was relatively level side-to-side) with a starting psi of no less than 59. In fact, the chart suggests that I could have run those front tires at 45-50 psi and they would not have been overloaded.

I am coming to the conclusion that something about the truck's TPMS is . Not only that, it's with little to no documentation in the owner's manual on how to adjust or override it.

Maybe it was just Saturday morning right after a blue moon?
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Old 08-04-2015, 06:39 PM   #28
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Ah... the "Eureka!" moment?

It may all come down to this, which I just found:

"Your vehicle has also been equipped with a TPMS malfunction indicator to indicate when the system is not operating properly. The TPMS malfunction indicator is combined with the low tire pressure telltale. When the system detects a malfunction, the telltale will flash for approximately one minute and then remain continuously illuminated. This sequence will continue each time the vehicle is restarted as long as the malfunction exists."

When I started the truck up again after a break of probably half an hour (and reducing the pressure in those front tires by a couple of psi), the warning disappeared and never happened again. I don't remember it saying anything about a "malfunction," but the flashing with the low tire pressure warning, followed by continuous illumination is precisely what happened.

I think it's time to schedule a service appointment at the dealership to have it checked out.
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