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Old 05-24-2016, 11:51 AM   #1
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Exclamation Tire Pressure

I have a 2015 Forest River Georgetown 378XL. The recommended tire pressure is 100 lbs psi when cold. I have equipped it with an automatic tire pressure and temp reader on all six wheels. It is impossible to inflate the tires to 100 lbs on all wheels as the temp is never the same on each wheel. I have a 2 to 5 lb difference on each wheel. What am I doing wrong?

I have

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Old 05-24-2016, 12:34 PM   #2
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How are you inflating them?
Are you checking them cold? (Less than 3 miles running)

Are you using a handheld inflation gauge to check the pressure? If so, maybe you need a better gauge.

As for the temps, they're never going to be the same, the ones on the sunny side will always be warmer, but that shouldn't matter very much.

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Old 05-24-2016, 03:16 PM   #3
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Tire Pressure

I have a top of the line tire pressure gauge as well as the reading from tire pressure sensing unit. I have a large air compressor that I bring right to the motorhome. I inflate all tires to read 100 lbs. psi . However that is the last time I see a reading of 100 psi. When in use tires vary. How much variance should there be?
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Old 05-24-2016, 03:51 PM   #4
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Tire Pressure

What pressure monitoring system are you using? I fill my tires to 65 psi cold with a 12v Viair. The gauge on the pump reads 65. My digital gauge reads 65. My TST 507 system reads three tires at 64 cold and one at 66 cold. Very acceptable to me. I will see an increase across the board of 5 psi when tire temps are 90 degrees.
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Old 05-24-2016, 04:04 PM   #5
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There is a margin of error in most tire monitoring systems, however this is not a problem. The primarily function of a tire monitoring system is to detect fast or slow leaks or dramatic increase in temperature. I would not be concerned about small deviations in pressure. The pressure will vary from one side of the vehicle to the other based on which side is in the sun or shade. Also your pressure will rise as the tire heats up from normal driving or ambient temp changes.

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Old 05-24-2016, 04:07 PM   #6
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You ain't doing nuttin wrong . Three or five percent is pretty close . My tires pressures moves five percent with temperature changes .
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Old 05-24-2016, 04:31 PM   #7
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If you put 100 psi in the tires according to your pressure gauge then that's what you have. None of the TPMS systems are going to read exactly the same as the "gauged" pressure except by luck. But, they are close enough. What you are really looking for is variances and consistency in pressure changes as you drive. Don't worry about it.

Another thing. Gauge accuracy is all over the scale. I've seen differences of +/- 6-7 pounds when comparing the readings between a "quality" automotive gauge costing $50 or so vs a Tronair ASME TSO certified gauge costing nearly $400 (plus the annual cost of about $100 to certify accuracy). Close enough for our purposes. If you gauge them all at 100 psi then they are all the same. The "same" could be 97 psi or 104 psi.

If your TPMS started the day with all tires within five or so pounds of each other and they stay reasonably close to the variance then all is well. However, if you see five tires displaying 105 (or whatever) and the sixth one says 82 then it's time to pull over and find the problem. By pull over I mean as soon as it's safe to do so. It doesn't mean at the next exit 5-6 miles down the road.
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Old 05-24-2016, 05:05 PM   #8
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Tire Engineer here

Everyone above is giving good advice. Use your digital hand gauge for setting your pressures. The TPMS is a "Monitoring" device primarily intended to warn of pressure loss.

Ideally you should check and set pressure when tires have not been driven in last two hours. OR in direct sunlight for last two hours.

Stop at your tire dealer and ask them to compare their master gauge reading on a couple of tires that are inflated to a 90 - 110 psi range and compare your hand gauge. The gauges should match within +/- 3 psi or less. I wouldn't worry if you are within that on your hand gauge.
More variation than 5 and it's time to get a new gauge.

Once you set all your tires you will note the TPM readings and can see which one is highest and which is lowest. TPMS readings should be +/- 5 psi from the hand gauge.
Next morning when you use the TPMS to give the morning reading you should see similar spread of pressure.

With a Motorhome I suggest you set the low pressure warning level to the inflation needed to support the measured load of the heaviest position on each axle. Your Cold Inflation Pressure should be plus 10% of the minimum needed to carry the measured load based on the tables. so you have a nice margin for day to day variation due to changes in ambient temperature. Your pressure can vary day to day by +/- 5 psi and you will always have a nice margin if you use the above suggestion.

Tire pressure will change by about 2% for a 10F change in Ambient with air or N2.

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