What is correct tire pressure to inflate tow vehicle

[dkottum]

I agree with Slowmover, the place to start is with a CAT scale. Even a simple trip to the scale with the empty truck by itself, and another on your way out of town loaded and with the trailer hitched up will be useful.

You will know the weight each truck axle, and the trailer axles are carrying. You can compare that to the GAWR (maximum axle weight rating) on the truck and trailer placards on the vehicles. You can compare that to the maximum load embossed on the tire itself. You will know if you have overloaded any of these components.

You will also know if your weight distribution is set up to do its basic job, to restore the steering axle load to it's unloaded weight.

It can take some time to get w.d. set up ideally, but it's good to know what you're starting with.

[John Woodrow]

Originally Posted by CapriRacer

Here's the problem, John. To my knowledge, no one has ever done a full on handling test at different inflation pressures than those listed on the vehicle tire placard - unlike the vehicle manufacturers who not only do full on tests, but do them repeatedly with different evaluators driving. Without doing a full on handling test - one that destroys the tires - one can't be sure if the vehicle behaves benignly or not.

Nowadays, even before the first vehicle is tested, there is a computer simulation that is used to optimize things and look for potential problem areas. These computer simulations have been in use for over 30 years and are now quite accurate - to the point that Formula 1 teams (and others) use them to set up their vehicles - then verify that set up once the track is available.

I think that qualifies as highly sophisticated engineering calculations.

Based on your comments, you apparently set the pressures on your race car tires to the factory recommended specification and hit the track?
How is that working out for you? Last I knew, virtually all racers checked temperatures to find optimum pressure and correct alignment set ups.

I find it kind of surprising that given all of the sophisticated engineering calculations related to tire pressures, 32 psi seems to always be the answer.

[John Woodrow]

Agreed. Where is the testing data for your particular TV & TT combination, loaded? Don't have it? Well that makes you a test pilot! Heading off boldly into the unknown, I'd suggest starting with recommended tire pressures then adjust them as needed based on actual temperature readings across the tread.

[rocco52]

I am surprised that nobody mentioned going to the tire manufacturer website for info on pressure and loading. I weighed my rig at the scales and then used the axle loading to determine the correct pressure using the recommended 10% reserve capacity. Here is the link sent to me by Michelin Customer Service, oddly for Toyo Tires which must be a subsidiary.

https://toyotires-1524598101.netdna-...s_20170203.pdf

[slowmover]

Originally Posted by John Woodrow

Agreed. Where is the testing data for your particular TV & TT combination, loaded? Don't have it? Well that makes you a test pilot! Heading off boldly into the unknown, I'd suggest starting with recommended tire pressures then adjust them as needed based on actual temperature readings across the tread.

It's pressure rise that matters.

Temps are unreliable and inconsistent. They'll pretty much come down to, "my TV tires run higher (apparent) temps than my TT tires".

Measuring brake and bearing housing temps is a way of finding adjustment or wear issues. And that pretty well sums up the use of that tool in re road testing.

It's enough work to find the lowest useful pressure inside the manufacturer guidelines. On the hottest day. Under the heaviest load encountered. At a given cruise control setting.

Tire pressure isn't suspension tuning. But you'd never know it from listening to the RV crowd. They'll jack it high enough to make a bad situation worse (pickups)

[Tireman9]

Originally Posted by gldnpupper27

An Airstream dealer advised me to use the pressures on the drivers door for correct tire pressures, 35 PSI, even though the Max inflation on my Dynapro 275 55/R20 113T have a Max inflation pressure on the tire at 51 PSI. I have a Ford Expedition 2016.
Other forums seem to indicate that for full loads applied to the tow vehicle while towing, you should increase the tire pressure according to the manufacturer's load charts. We are about to embark on our maiden voyage soon and would appreciate your thoughts.

What does the owner's manual say for the Ford when towing? Do you have a HD towing package on the Ford?
Are the "Dynapro" tires OE? I'm not familiar with that line. Who makes the tires?

When hooking up a trailer many times the extra load along with gear you may have packed in the TV might put you over the GAWR for the rear axle.

Without knowing the actual scale load on the TV tires we can only guess and I assume that since you are asking the question you would want answers based on facts rather than guesses.

[SteveSueMac]

Hi all - been a while since I've been here - just checking in on this question following a trip to the scales this morning after having some custom work done on the trailer at JC earlier this year. I think the scale tickets suggest I can reduce pressure in both truck and trailer tires - truck to ~65PSI on all 4 tires and trailer to ~60-62 on all 4 tires and I welcome your thoughts based on the data and details below:

Here are the ticket #s with TV/TT fully loaded for camping including gear in bed of truck, full tank of diesel, full LP and FW tank on trailer and gear in trailer for camping:

Truck Only:
Steer 4660
Drive 3880

Trailer attached no WD (using a ProPride, swinging freely)
Steer 4140
Drive 5520
Trailer 5960
(Derives 1120 tongue weight - 80# heavier than last year)

Trailer with 7" WD on the jacks (used to use 6, but tried 6 and didn't like scale numbers so using 7)
Steer 4540 (400 restored of 520 lifted or 77% FALR; GM wants 50% or better)
Drive 4980
Trailer 6120

I have GYEs (ST225/75R15 LRE) on the trailer and Michelins (I forget the exact numbers - they're 20" LRE, I think 275/65R20 but LRE and the charts for those are common).

So with less than 5000# on each truck axle (2500/tire) I could in theory go down to 50 PSI which would give each tire 2680# of capacity according to the load chart. This just intuitively feels hard to believe. But if I look at 3030# per tire or 6060# per axle (some 17ish% headroom) I'm looking at 60 PSI. Better, but I'm still leery - I'm thinking 62-65 PSI may the right balance for the truck tires.

For the trailer, its total weight is 7100#. When connected to the truck and with its tongue weight distributed across 3 axles, the tires are seeing 6120# on the road (unless I'm seeing that incorrectly). The GYE load chart for these tires suggests and if the trailer were 8000#, I could be using 45 PSI to get 2020#/tire or to cover 8000# (I don't have individual wheel weights which would be awesome, so I know I have to add a fudge factor). Again, 45 PSI on the trailer tires seems silly! But even to carry a 10,000# trailer, 70 PSI (their max is 80) would cover 2620#/tire and a full 10000#. I've been running them at 72 and they seem to be ok - not a significant rise in pressure or temps according to the TPMS but some evidence of rough ride inside the trailer when getting to campgrounds. But I'm wondering if 60-65 (65 is on the placard, but originally it was GYMs, not GYEs on the trailer and they were LRD) is a fair range in your view?

I'm particularly interested in lowering the pressure (if possible) because I'm assuming it will ease up the ride a bit, which unfortunately due to new spinal stenosis/arthritis is sometimes unbearable, especially on poorly maintained roads. While there are many other options I'm pursuing (new tow vehicle, air ride after market driver's seat if possible, etc.) I'm asking this question specifically and only about tire pressures based on my scale numbers and some planned long trips between now and March 2022.

To sum - the scale tickets suggest I can reduce pressure in both truck and trailer tires - truck to ~65PSI on all 4 tires and trailer to ~60-62 on all 4 tires. What do you think?

Thanks,
steve

[Tireman9]

Originally Posted by CapriRacer

I spent 5 years as the technical rep to Ford for a major tire manufacturer. I worked along side of people who called on GM, Toyota, etc.

They all do it the same way. They do handling tests on everything they make - every model car and every model truck, both empty and fully loaded at the pressure listed on the vehicle tire placard. They know from experience that there is a small difference between tire manufacturers at a given pressure and that pressure matters much more than brand. They will request changes in the tire to get the feel that they want, but these are minor tweaks. Plus they will also report to their management if the vehicle is doing odd things (in other words, needs sway, spring rate and/or shock changes.)

And the big thing I learned about this testing is that they did not deviate from the pressure listed on the vehicle tire placard - and they only approved a vehicle for production if it didn't do anything unpredictable.

So I would be very careful deviating from what the vehicle tire placard says.

x2 from the other tire design engineer here (different company).

[Tireman9]

Originally Posted by John Woodrow

Agreed. Where is the testing data for your particular TV & TT combination, loaded? Don't have it? Well that makes you a test pilot! Heading off boldly into the unknown, I'd suggest starting with recommended tire pressures then adjust them as needed based on actual temperature readings across the tread.

John, Are you using a tire needle probe to learn the temperature at the critical location in a tire? 3 readings per tire with all tires measured within a minute of stopping quickly from full highway speed? (as we do with race cars)

[SteveSueMac]

Well - let me answer your question, SteveSueMac….

Great - thanks for chiming in, SteveSueMac! :-)


Took the rig out this weekend - mostly a highway drive, though some crappy maintenance left for no shortage of potholes and other filling removal devices.

Truck and trailer handled things really well. Using my TPMS for my trailer, I noted an 8-10% rise in pressure and +30 degree rise over ambient temps - this seems within the range I recall from earlier discussions with the tire experts on this forum.

The truck TPMS (native to the truck) only shows pressure. Its pressures rose a little less than that - 6-8% - I don’t have a thermometer to measure temps but by hand none of them felt excessively warm.

I did add another 1/2” (so now up to 7.5”) on the WD bars for the ProPride and the ride got just a bit more stable on the way home than on the way up - less porpoising (my scale weights at 7” don’t have the front/rear axles of the truck as close to even as I’ve had in the past so this may have helped).

Overall, I think I’m pretty satisfied with the slightly lower pressures (all of which have significant headroom over the scale weights and load charts) - smoother ride, no shocks up the spine, no stuff on the floor of the camper (and we went over a few doozies too - all good!).

Wide open to after-the-fact comments/insights as well. Thanks.

Originally Posted by SteveSueMac

Hi all - been a while since I've been here - just checking in on this question following a trip to the scales this morning after having some custom work done on the trailer at JC earlier this year. I think the scale tickets suggest I can reduce pressure in both truck and trailer tires - truck to ~65PSI on all 4 tires and trailer to ~60-62 on all 4 tires and I welcome your thoughts based on the data and details below:

Here are the ticket #s with TV/TT fully loaded for camping including gear in bed of truck, full tank of diesel, full LP and FW tank on trailer and gear in trailer for camping:

Truck Only:
Steer 4660
Drive 3880

Trailer attached no WD (using a ProPride, swinging freely)
Steer 4140
Drive 5520
Trailer 5960
(Derives 1120 tongue weight - 80# heavier than last year)

Trailer with 7" WD on the jacks (used to use 6, but tried 6 and didn't like scale numbers so using 7)
Steer 4540 (400 restored of 520 lifted or 77% FALR; GM wants 50% or better)
Drive 4980
Trailer 6120

I have GYEs (ST225/75R15 LRE) on the trailer and Michelins (I forget the exact numbers - they're 20" LRE, I think 275/65R20 but LRE and the charts for those are common).

So with less than 5000# on each truck axle (2500/tire) I could in theory go down to 50 PSI which would give each tire 2680# of capacity according to the load chart. This just intuitively feels hard to believe. But if I look at 3030# per tire or 6060# per axle (some 17ish% headroom) I'm looking at 60 PSI. Better, but I'm still leery - I'm thinking 62-65 PSI may the right balance for the truck tires.

For the trailer, its total weight is 7100#. When connected to the truck and with its tongue weight distributed across 3 axles, the tires are seeing 6120# on the road (unless I'm seeing that incorrectly). The GYE load chart for these tires suggests and if the trailer were 8000#, I could be using 45 PSI to get 2020#/tire or to cover 8000# (I don't have individual wheel weights which would be awesome, so I know I have to add a fudge factor). Again, 45 PSI on the trailer tires seems silly! But even to carry a 10,000# trailer, 70 PSI (their max is 80) would cover 2620#/tire and a full 10000#. I've been running them at 72 and they seem to be ok - not a significant rise in pressure or temps according to the TPMS but some evidence of rough ride inside the trailer when getting to campgrounds. But I'm wondering if 60-65 (65 is on the placard, but originally it was GYMs, not GYEs on the trailer and they were LRD) is a fair range in your view?

I'm particularly interested in lowering the pressure (if possible) because I'm assuming it will ease up the ride a bit, which unfortunately due to new spinal stenosis/arthritis is sometimes unbearable, especially on poorly maintained roads. While there are many other options I'm pursuing (new tow vehicle, air ride after market driver's seat if possible, etc.) I'm asking this question specifically and only about tire pressures based on my scale numbers and some planned long trips between now and March 2022.

To sum - the scale tickets suggest I can reduce pressure in both truck and trailer tires - truck to ~65PSI on all 4 tires and trailer to ~60-62 on all 4 tires. What do you think?

Thanks,
steve

[JSMITH8505]

Lets put the "Door sticker/tire pressure" myth to rest.



As a tire manufacturing engineer once told me "Does your door sticker know what tire you're using?" *(see last paragraph)



The door sticker on your tow vehicle only tells you the recommended pressure for the OEM tire during your vehicle's production year.

It is 100% dependent on the tire technology for that era, the brand, and it's usage.



For example, a 1976 Mercedes 300D from my childhood showed a 28 psi rating on the door sticker. I remember, because as inquisitive 11 year old who was into all things mechanical, The tire side wall of that era showed a max psi of 35. My dad found that 32 psi gave a good ride, good tire wear and predictable rain/dry pavement driving. 28 psi "wallowed" in turns and "squished" on the highway making the car feel sloppy and unpredictable. At 25 mph it was great as it floated over bumps, but at highway speeds, forget about planting the car where you wanted to in a lane-changing pass.



Fast forward to today- The tires are stamped with varying "MAX load capacity AT xx psi" ratings. They vary by brand, type (off-road/all terrain/highway/winter/snow) ratings.



**A former neighbor was an engineer for a tire manufacturer and when I brought up this very topic (door sticker/sidewall max psi rating) he gave me that saying "Does you sticker know what tire you're using?"



Unless your door sticker 'knows' what tire/version/generation you're currently driving on, you should rely strictly on the max psi rating value of the sidewall of your tire. Adding and deducting based on weight, temperature and speed.



Each tire manufacturer worth it's salt posts speed ratings on their website for each tire. some of the better ones actually show what adjustments should be made for various load/speed situations.

[JSMITH8505]

One last comment and then I'm done:

As one who tows 100k plus miles per year, I have to keep careful watch on tire wear.


On my Ram 3500 Dually (80 psi front/65 psi rear) there are no downfalls to towing at max psi.



Again, There is no real penalty (overall life of tire, load handling capacity, etc) in running at Max PSI. The bonuses are that you get better temperature resistance and better wear, and perhaps a bump in fuel economy due to lower rolling resistance.



The problems seem to occur when you decrease the psi.

Weight wise- (loaded weight) there is no benefit for dropping tire pressure below max. It *seems* it's all for comfort. And If you drop just a little too much then you'll actually start seeing detrimental issues. (heat/friction, uneven wear, lower load capacity).



The key is to not exceed the rim/wheel failure rating. Keep your psi below the failure rating (available on rim mfg websites) and you'll be fine.



Towing is an entirely different scenario than any other driving type.



Race cars need sticky tires that flex enough to grip in turns, kind of like court shoes for tennis or basketball.



Tow vehicles need sufficient sidewall strength and a lack of flex so that the tire doesn't "roll" over. Kind of like work boots for those who lift and pull heavy pipe on an oil rig. Put a running shoe on this person and they're foot will roll over and they lose any step-generated power they have.

[Tireman9]

Originally Posted by JSMITH8505

Lets put the "Door sticker/tire pressure" myth to rest.
As a tire manufacturing engineer once told me "Does your door sticker know what tire you're using?" *(see last paragraph)
The door sticker on your tow vehicle only tells you the recommended pressure for the OEM tire during your vehicle's production year.
It is 100% dependent on the tire technology for that era, the brand, and it's usage.
For example, a 1976 Mercedes 300D from my childhood showed a 28 psi rating on the door sticker. I remember, because as inquisitive 11 year old who was into all things mechanical, The tire side wall of that era showed a max psi of 35. My dad found that 32 psi gave a good ride, good tire wear and predictable rain/dry pavement driving. 28 psi "wallowed" in turns and "squished" on the highway making the car feel sloppy and unpredictable. At 25 mph it was great as it floated over bumps, but at highway speeds, forget about planting the car where you wanted to in a lane-changing pass.
Fast forward to today- The tires are stamped with varying "MAX load capacity AT xx psi" ratings. They vary by brand, type (off-road/all terrain/highway/winter/snow) ratings.
**A former neighbor was an engineer for a tire manufacturer and when I brought up this very topic (door sticker/sidewall max psi rating) he gave me that saying "Does you sticker know what tire you're using?"
Unless your door sticker 'knows' what tire/version/generation you're currently driving on, you should rely strictly on the max psi rating value of the sidewall of your tire. Adding and deducting based on weight, temperature and speed.
Each tire manufacturer worth it's salt posts speed ratings on their website for each tire. some of the better ones actually show what adjustments should be made for various load/speed situations.

I don't know anyone towing an RV with a 1976 MB car. Today most tow with a 3/4 ton or bigger Pickup running LT type modern radials. The door sticker and owner handbook should tell you the tire size, Load Range and recommended inflation. Many also show the recommended inflation adjustment it towing.
If you are running the tire type/size shown on the sticker then that is an appropriate inflation as long as you do not exceed to towing capacity specified in the owner's manual.
If/when you change tires it then becomes the owner's responsibility to determine the appropriate inflation based on measured loads.


The tire does not know what vehicle it is on, or what load the owner is placing on the tire. The Max load number on the tire is just that. The inflation on the tire sidewall is also the inflation needed to support that load. You can run higher inflation but that does NOT translate to any increase in load capacity.
The Industry standardized Load/Inflation tables make no allowances for "type (off-road/all terrain/highway/winter/snow)" If you can direct me to any outliers I would be interested.


I am a tire design engineer and wonder what tires that other "engineer" designed but his/her statement is basically correct but does not address the situation.

[SteveSueMac]

Originally Posted by Tireman9

…snip…

If/when you change tires it then becomes the owner's responsibility to determine the appropriate inflation based on measured loads.

…snip...

OEM tires on my truck were Wranglers which I changed to Michelins (but same exact size and load rating). OEM tires on my trailer were GYM STs load range D with a sticker asking for 65 PSI (max pressure) which I’ve changed to GYEs with load range E.

If I understand you correctly you’re saying it’s my responsibility to determine inflation based on measured loads. Would you care to react to my post in #27 above which outlines my thought process on running the pressures I am based on measured loads? I welcome your expert insights….


Thanks!