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Old 01-17-2019, 12:13 AM   #1
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Tire Inflation Question

I think I know the right answer to this but I want to check myself out here (keyword 'think').

The sticker on the door jam says 75 front 80 rear. The tires are 75 all around, give take 1 or 2.

Question: did someone under inflate the rear on purpose to leave room for expansion?
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Old 01-17-2019, 12:46 AM   #2
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The sticker is the recommended tire pressure if you are running fully loaded - at your GVWR... It is safe to use at any weight - but may result in a harsher ride if lightly loaded. It is a 'cold' pressure and accounts for expansion and higher pressures that will result from traveling.
Getting your rig weighted in traveling conditions (cargo, fluids including gas, propane, water, people) to get axle weights (CAT Scales at truck stop as example) - and checking the tire manufacturers website for inflation recommendations at that weight will give you another data point - safe to use at that weight range.
Many threads on the subject...
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Old 01-17-2019, 03:19 AM   #3
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Originally Posted by gmc View Post
The sticker is the recommended tire pressure if you are running fully loaded - at your GVWR... It is safe to use at any weight - but may result in a harsher ride if lightly loaded. It is a 'cold' pressure and accounts for expansion and higher pressures that will result from traveling.
Getting your rig weighted in traveling conditions (cargo, fluids including gas, propane, water, people) to get axle weights (CAT Scales at truck stop as example) - and checking the tire manufacturers website for inflation recommendations at that weight will give you another data point - safe to use at that weight range.
Many threads on the subject...
Thank you. Got it
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Old 02-02-2019, 05:22 AM   #4
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Originally Posted by gmc View Post
..Getting your rig weighted in traveling conditions (cargo, fluids including gas, propane, water, people) to get axle weights (CAT Scales at truck stop as example) - and checking the tire manufacturers website for inflation recommendations at that weight will give you another data point - safe to use at that weight range.
Many threads on the subject...
Had it weighed. 400 over on the GVWR somewhere. Had a full tank+ (to the brim) and 1/3 fresh water. Can't discard anything and all OHC half empty. Not sure where it's coming from. Under GCWR by 4k+ lbs. 4900 on the front, 10k on the rear, 3560 on the towed (picked up weight somewhere here too) - 18460 overall.


Not exactly sure how to read the tire inflation chart. Once saw this for the same tire rated at 9880 for D. Seems I'm little light up front and heavy out back. Wonder if I positioned myself right on the scales.

Don't want to be anal but should I be 'worried'?
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Old 02-02-2019, 10:54 AM   #5
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Quote:
Originally Posted by TheBreeze View Post
Had it weighed. 400 over on the GVWR somewhere. Had a full tank+ (to the brim) and 1/3 fresh water. Can't discard anything and all OHC half empty. Not sure where it's coming from. Under GCWR by 4k+ lbs. 4900 on the front, 10k on the rear, 3560 on the towed (picked up weight somewhere here too) - 18460 overall.


Not exactly sure how to read the tire inflation chart. Once saw this for the same tire rated at 9880 for D. Seems I'm little light up front and heavy out back. Wonder if I positioned myself right on the scales.

Don't want to be anal but should I be 'worried'?
I would be, spent too many years pulling TTs too close to the tires' load ratings. I always replaced with higher load rated tires for more safety margin.
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Old 02-02-2019, 11:52 AM   #6
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Firmly believe in over spec’ng Equipment particularly tires.
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Old 02-02-2019, 12:50 PM   #7
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Firmly believe in over specíng Equipment particularly tires.
Same here

And, no it doesn't matter where you stop on the scale if the axles you are weighing are completely on the platform
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Old 02-02-2019, 02:39 PM   #8
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"Question: did someone under inflate the rear on purpose to leave room for expansion?"

Tire pressures are always measured when cold, and expansion due to heat build-up has already been taken into account...
They probably went to 75 psi on all four tires for the purpose of saving some time...
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Old 02-02-2019, 03:57 PM   #9
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Load ratings

I think you should just believe the posted load ratings for the tires you have. The manufacturer has already taken into account different road conditions when coming up with their numbers. The safety margin is built into the numbers. When measuring your own rigs weight at a weight station, your towed weight' tongue load needs to be added to the rear weight for a true rear weight load. Are you sure you are not carrying some gold bars for an emergency. They are very heavy.
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Old 02-02-2019, 04:10 PM   #10
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I think you should just believe the posted load ratings for the tires you have. The manufacturer has already taken into account different road conditions when coming up with their numbers. The safety margin is built into the numbers. When measuring your own rigs weight at a weight station, your towed weight' tongue load needs to be added to the rear weight for a true rear weight load. Are you sure you are not carrying some gold bars for an emergency. They are very heavy.

You definitely never want to run lower pressures than the name plate. As the others have said run it across the scale. You need to run both sides of the axle at the same pressure necessary to match your actual loads.

Many tire sidewalls are ruined by running them to low. In my experience running the tire side wall value has worked fine although many folks like to have the tire cushion the ride a bit. I like it a bit firmer and don't like the squirm you get running them lower.

Bottom line is if they run hot then you are either under inflated or over loaded and if you are at 130 already then hopefully they are not running hot.

Be sure to let us know what you come up with after weighing the axles
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Old 02-03-2019, 01:32 AM   #11
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Originally Posted by TheBreeze View Post
Had it weighed. 400 over on the GVWR somewhere. Had a full tank+ (to the brim) and 1/3 fresh water. Can't discard anything and all OHC half empty. Not sure where it's coming from. Under GCWR by 4k+ lbs. 4900 on the front, 10k on the rear, 3560 on the towed (picked up weight somewhere here too) - 18460 overall.


Not exactly sure how to read the tire inflation chart. Once saw this for the same tire rated at 9880 for D. Seems I'm little light up front and heavy out back. Wonder if I positioned myself right on the scales.

Don't want to be anal but should I be 'worried'?
My opinion, some of the 'C's are shipped too close to their GVWR - not leaving room for reality...
You are 400 over now - but I would bet those OHC (and basement) will get more 'stuff' added as you go. And if you travel with more passengers than at weighing...
You will need to be very conscious of what you pack. Move some heavier 'stuff' to your toad - as you have GCWR to give (moves weight off the RV tires/suspension)
You certainly want your tires at the placard pressure at that weight.
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Old 02-03-2019, 01:45 AM   #12
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The tires themselves should have the "max weight @ xxxpsi" on the sidewall. Once you know this, then check to see what weight is on tires front and rear. You may be over the GVWR but under the tire capacity--or not...
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Old 02-03-2019, 02:17 AM   #13
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What model chassis and coach is this?
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Old 02-03-2019, 04:56 AM   #14
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Quote:
Originally Posted by Yaktop View Post
What model chassis and coach is this?
I 'd like to thank everyone for their help.To quote each of you and personally thank you would make this post verbose. I'll refrain. You have to love this place. Thank you all.

The chassis is an extended E450 for a 2017 Thor Chateau at 33'. The tires are shown. While I agree with some to overcompensate on load ratings, the tires are brand new. However, I would be a fool to waste the RV to save some tires. Load rating is E. They don't seem to make the size in a higher capacity?

Thanks again.
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Old 02-03-2019, 05:24 AM   #15
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Quote:
Originally Posted by gmc View Post
My opinion, some of the 'C's are shipped too close to their GVWR - not leaving room for reality...
You are 400 over now - but I would bet those OHC (and basement) will get more 'stuff' added as you go. And if you travel with more passengers than at weighing...
You will need to be very conscious of what you pack. Move some heavier 'stuff' to your toad - as you have GCWR to give (moves weight off the RV tires/suspension)
You certainly want your tires at the placard pressure at that weight.

I would normally agree however, I could have gotten away with much less room than I have. Who knew? I bought this rig for reasons I believed were valid at the time. I could have gotten away with less. It is what it is.

I get your point on moving stuff to the towed. I have to find a bed cap to do that. I've been passively looking and it's probably time to pull the trigger.

Thanks for the feedback. Take care and safe travels.
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Old 04-06-2019, 03:04 PM   #16
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What are max temperatures for tires and where can that be found?

My electric pressure detector also monitors temperature. Where do I get those figures?
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Old 04-06-2019, 03:51 PM   #17
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Originally Posted by jgannone@gmail.com View Post
My electric pressure detector also monitors temperature. Where do I get those figures?
From the tire manufacturer. The side wall letter on passenger tires shows the temperature rating but light truck tires do not. It is safe to assume that all properly inflated tire should run below 150 F degrees, but usually the manufactures will specify 180 as the max for tire life. Side wall separation will occur at temps above 200 F degrees. For me. I find tire pressures vary much faster than tire temperatures. A rapid tire pressure drop of 25 psi will immediately trigger the TPMS alarm where as the temperature rise of 20 degrees needed to trigger the alarm will take a minute or more. My front tires run 98 psi with a cold pressure of 78 and the rears run 114 psi with a cold pressure of 88 psi on a 100+ day at 60 mph.
Tire pressure goes up with ambient temps and speed (around corners or straight line).
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Old 04-06-2019, 04:40 PM   #18
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Many thanks sir. I hope to return the favor someday.
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Old 04-06-2019, 08:43 PM   #19
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I found the placard stating tire pressures. It says front and rear 80psi. Does that sound right for a 32 foot Thor?
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Old 04-07-2019, 03:49 AM   #20
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Originally Posted by jgannone@gmail.com View Post
I found the placard stating tire pressures. It says front and rear 80psi. Does that sound right for a 32 foot Thor?
For my 18,000 lb chassis, the door sticker shows 82 psi all around on the 245/70-19.5 tires. After weighing the coach and noting the increase in tire pressures during trips I try to run 78 psi in the front and 88 psi in all four rears.
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