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Old 11-16-2022, 02:04 PM   #1
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Tire air pressure

Looking for recommendations about tire air pressure. I have a 2006 E350 6.0 R50 with Sportsmobile 4x4, tire size LT 315/70R17 121/118R M+S. Anyone know what pressure to run in the front and back? I guess it's different for off road and freeway? Thanks
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Old 11-16-2022, 02:44 PM   #2
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Here is one:
https://www.sportsmobileforum.com/fo...les-28655.html

I can't recall who it was, but I'm almost positive this came from a Forum member here - thought it was good info so copied it:

TIRE PRESSURE
Chapter 1 : How to read tire information?

There is quite a bit of information on the sidewall of a tire:

Metric example: 195/65 R 15 91H M+S
1 – overall width of the tire [mm]
2 – height of side wall [% of global width]
3 – indicates that it’s a Radial Carcass (when Diagonal ? -)
4 – diameter of the rim [inch]
5 – load index
6 – speed index
7 – brand or type
8 – DOT : indicates outside sidewall of the tire and date of production
9 – Tubeless (TL) or Tubetype (TT)
10 – M+S : Mud + Snow, indicates that tire is appropriate for snow conditions

Imperial example: 35/12.5 R 17 125Q
Only numbers 1 and 2 change:
1 – overall diameter of the tire unloaded [inch]
2 – overall width of the tire [inch]

Certain tires show some letters directly ahead of first size indication, for example, LT245/70 R 17.
That indicates for which vehicle category they are designated.
P : Passenger vehicle (cars)
LT : Light trucks
etc.

Certain tires also show maximum inflation pressure allowed, which depends of tire construction. Be careful: NEVER ECXEED THIS MAX. PRESSURE ! The bursting risk is real..!

Chapter 2 : How to choose the right tires for my rig?

They are basically four criteria to consider:
- maximum speed of the vehicle
- maximum weight of the vehicle
- width of the rim
- diameter of OE tires

Two first criteria determine the minimal load x speed index of the tire. Legally, you must have tires with speed index at least equivalent to the maximum speed of your vehicle.
Here is the speed index list:

Speed Index - Mph - Km/h
L--------75 mph-----120 km/h
M--------81 mph-----130 km/h
N--------87 mph-----140km/h
P--------93 mph-----150 km/h
Q--------99 mph-----160 km/h
R-------106 mph-----170 km/h
S-------112 mph-----180 km/h
T-------118 mph-----190 km/h
U-------124 mph-----200 km/h
H-------130 mph-----210 km/h
V-------149 mph-----240 km/h
W------168 mph-----270 km/h
Y-------186 mph-----300 km/h
Z----->186 mph---->300 km/h

Important is to understand that these speed limits are for the tire at nominal inflation pressure which is pressure for maximum acceptable load on the tire.

So, if our SMBs’s maximum speed is 90 mph, we can mount every tire with speed index P or higher.

Second tire index is the maximum acceptable load per tire. Legally, you must have tires with load index adequate for the maximum weight of the most loaded axle at maximum admissible weight (GVWR) of your vehicle. For example, if the GVWR is 9,500 lbs with 4’000 lbs on front axle and 5,500 lbs on rear, minimum load index must correspond to 2,750 lbs.
Here is the nominal load index list which indicates maximum load at maximum admissible speed:

Load Index - Pounds - Kilograms
71--------761---------345
72--------783---------355
73--------805---------365
74--------827---------375
75--------853---------387
76--------882---------400
77--------908---------412
78--------937---------425
79--------963---------437
80--------992---------450
81-------1019---------462
82-------1047---------475
83-------1074---------487
84-------1102---------500
85-------1135---------515
86-------1168---------530
87-------1201---------545
88-------1235---------560
89-------1279---------580
90-------1323---------600
91-------1356---------615
92-------1389---------630
93-------1433---------650
94-------1477---------670
95-------1521---------690
96-------1565---------710
97-------1609---------730
98-------1653---------750
99-------1709---------775
100------1764---------800
101------1819---------825
102------1874---------850
103------1929---------875
104------1984---------900
105------2039---------925
106------2094---------950
107------2149---------975
108------2205--------1000
109------2271--------1030
110------2337--------1060
111------2403--------1090
112------2469--------1120
113------2535--------1150
114------2601--------1180
115------2678--------1215
116------2755--------1250
117------2832--------1285
118------2910--------1320
119------2998--------1360
120------3086--------1400
121------3196--------1450
122------3306--------1500
123------3417--------1550
124------3527--------1600
125------3637--------1650
126------3747--------1700
127------3858--------1750
128------3968--------1800
129------4078--------1850
130------4387--------1990
131------4299--------1950
132------4409--------2000
133------4541--------2060
134------4673--------2120
135------4806--------2180
136------4938--------2240
137------5070--------2300
138------5202--------2360
139------5357--------2430
140------5511--------2500
141------5676--------2575
142------5842--------2650
143------6007--------2725
144------6172--------2800
145------6393--------2900
146------6613--------3000
147------6779--------3075
148------6944--------3150
149------7165--------3250
150------7385--------3350
151------7605--------3450
152------7826--------3550
153------8046--------3650
154------8267--------3750
155------8542--------3875
156------8818--------4000
157------9094--------4125
158------9369--------4250
159------9645--------4375

The speed index chart and the load index chart usually correspond to each other: for example 121P indicates that the tire supports 3,196 lbs at 93 mph.

You will also often see two different load index ratings on the same tire: 121/118P. The first one indicates load index when used on a single wheel axle and the second indicates maximum load on a dually axle.

A third criterion is the width of your rim. Each tire can be mounted on a rim width range indicated by the manufacturer. For example, a Toyo LT245/75R16/E 120R can be mounted on rims from 6.5’’ to 8.0’’ width. To mount the tire on smaller or wider rims won’t allow the sidewall to work properly and will degrade dynamic properties of the tire.

And fourth criterion is the radius of the tire under load. Every country law describes a maximum difference between speed indicator and real speed. So, if you change tire radius, your true speed will be off unless you change the transmission ratio of the speedometer to compensate the difference.

Speed index, load index and radius under load are generally legally enforced. Width of the rim is a producer specification.

Chapter 3: Which is the right inflation pressure of my tires?

Inflation pressure depends of many factors: tire characteristics, volume of the tire, load, speed, etc. And inflation pressure also directly affects dynamic properties of the tire, with influence on vehicle behavior and safety.

Here a reminder of basic principles:

- The more the vehicle weights and how fast the vehicle is traveling, dictates the tire pressure. Higher speeds and/or heavier vehicle weights require higher tire pressures. That’s why auto manufactures often show two recommendations of inflation pressure for a same car: the first for normal use (1 driver, one passenger), the second for heavy load and/or highways.

- Pressure applied by the tire on the ground doesn’t depend of the weight of the vehicle, but only of the inflation pressure! Many people struggle to understand this but you must remember that tires are flexible. So if the load increases, the tire(s) deform(s) and its contact surface with the ground also increases. But the pressure under this surface stays the same: always the inflation pressure. That’s why we deflate tires on soft grounds, reducing pressure under the tire to avoid becoming stuck.

- Inflation pressure depends of inflation volume of the tire. An example:
Original tire size of a Ford E350 is LT 245/70 R 16 on 7.0’’ rim. Supposedly they should be inflated at 60 Psi. If I replace this tire with a 35/12.5 R 17 on a 9.0’’ rim without changing the weight of the rig, inflation pressure in these larger tires will be lower.

- Maximum allowed speed decreases with lower inflation pressure. As I already said, it’s possible and recommended to deflate tires on soft grounds. But when done, you cannot drive as fast as with normally inflated tires. So when you leave soft ground, don’t forget to inflate your tires. If you don’t do it, your tires will warm up quickly and may burst. At a minimum, you can damage the integrity of the tire leading to premature failure.
Contrariwise, if you limit the speed lower than speed limit, you can exceed maximal load of the tire without risking bursting. There are specific charts indicating relationship between speed reduction and additional load allowed. Typically, a speed reduction of 10% allows a surcharge of 5%. But every tire manufacturer has its engineered ratings and specifications.

So, how to determine the right pressure for my rig?

First to do is to weigh your rig. Ideally, you should weigh it empty and then loaded (stuff and passengers). If you cannot do both, you must weigh it loaded. Important is to weigh each axle separately.
When done, you must consider tire size and type and refer to manufacturer’s inflation charts. Remember, it not only depends of tire size and weight, it also depends of construction type of the tire. So, even for the same size and same manufacturer, you can have different inflation pressures depending on tire type.

Chapter 4: When should I monitor tire pressure?

Recommended inflation pressure is always the cold tire pressure! Since a tire is flexible, when you drive, it warms up; more if you drive fast, less if you drive slow. So the only reference to check pressure is when the tire hasn’t been driven for two hours or more.
But what does it means?
1 – I only adjust pressure after the vehicle has set, (ideally in the morning before starting the vehicle).
2 – During cold weather, provided the vehicle spends the night in a heated garage with a large difference between inside and outside temperatures, you should take it outside and allow the tires to cool for half an hour before checking pressure.

If you have to adjust pressure after driving – for example when you drove a trail with deflated tires and you come back on a highway – overinflate your tires of about 5 Psi - 0.3 bar (it’s the estimating pressure increase due to the heat) and recheck cold pressure at the first opportunity.

And remember: it’s impossible to check tire pressure without a pressure gauge. It’s almost impossible to recognize a 10% underinflated tire from a well inflated one!

Chapter 5: Why is the right inflation pressure so important?

They are many reasons why inflation pressure is so important.

Security aspects
Too low of a pressure makes the tire warm up and can lead to rupture. Since a punctured tubeless tire loses air slowly, it’s important to check pressure regularly to detect a punctured tire. I’ve seen tire bursting after only 3 miles drive! An underinflated tire also causes a significant loss of road handling, steering precision and cornering stability since it deflects more as it rolls.
An over inflated tire has less longitudinal and lateral adherence. So breaking distances will increase with over inflation and traction will be lower. Ride will also be harsher and puncture risk is higher.

Longevity aspects
Many people fail to realize how much underinflating or over inflating a tire affects its longevity. A 5% over inflation can produce a loss of more than 15% in overall mileage of the tire, and 10% over inflation more than 30%. If the right pressure is 60 Psi, 10% is only 6 Psi..! Some people say “I overinflate my tires to provide better gas mileage”. They are right: by a 10% over inflation, you can hope to gain 3-5% better mpg. The problem is tire wear has also increased by 30% so the tires life is shortened. In the end is it worth saving a little fuel money compared to how long the tires last?

It’s about the same with under inflation even though it’s a little less severe regarding tire life: a 10% under inflation will affect tire life of about 20%. But remember: under inflation is very dangerous for your security!


I hope it will raise your awareness of regular monitoring of tire pressure… ;-)
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Old 11-16-2022, 07:32 PM   #3
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First you need to go weigh your rig, you want front axle weight and rear axle weight. This will allow you to figure out the weight per tire. Your load index of 121 gives you a weight rating of 3196 lbs. at assuming your load range is "E" 80 PSI.

Generally there are charts provided by the manufacturer to tell you what PSI to run for your weight based on your tire. You could also take load / max PSI (3196/80 = 39.95) Now take your per tire measured weight Axle/ 2 and then divide that by the calculated 39.95 and get your PSI by weight measurement.

Any printed load information is preferable to this calculation. I usually added 10% margin on the calc and went from there.

Somewhere on the forum is a excel calculator I made ,

https://www.sportsmobileforum.com/fo...p-25071-2.html
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Old 11-17-2022, 07:15 AM   #4
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That is a lot of great and accurate info given above. I'll give a very simple answer in contrast based on lots of vans and terrain over the years.

You didn't say but I'm guessing you have E-rated tires on your SMB. If they are D's you'll likely be fine but I'd make sure E's are on it at next tire change.

E-rated tires are VERY tough and a perfect match for the typical loaded out SMB at +/- 10k lbs. All that said I set my tire pressure for comfort. Typically 55 front/60 rear provides a nice ride in a heavy van and is a good compromise between comfort and efficiency (mpg's). Offroad I'll drop as low as 20 and on road if for some reason I'm heavier than usual (more people, gear, towing) I might bump it up.

It's a shame that so many people, especially in commercial applications, deal with the standard 80 psi crap that companies and tire shops go by. This not only kills your ride but makes the handling of the van a lot worse, as in twitchy, in my experience.

Two things that are more important than running pressure a little high or low are checking your pressure on a regular basis and driving the van. Tires do not like to sit for months on end. It drastically shortens their life.

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Old 11-17-2022, 12:43 PM   #5
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55F/60 R for me as well. 10,000lb rig on e rated 4080/80psi tires.
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Old 11-24-2022, 12:50 PM   #6
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What they said ! Good stuff here I think.

One thing though - over inflation can be as problematic as under inflation though less dangerous particularly in summer conditions. Over inflation decreases the size of the already small footprint of the tire and increases the tire sidewall rigidity. Both contribute to a lack of grip which particularly in wet or cold weather will promote oversteer or a loose handling. So if you enjoy hanging the tail out there up your calculated correct load pressure by this percentage or that percentage (many opinions out there) but its not without trade offs.
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Old 11-24-2022, 01:18 PM   #7
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Quote:
Originally Posted by JoeH View Post
55F/60 R for me as well. 10,000lb rig on e rated 4080/80psi tires.

Same here. 11,500 lbs. Falken wildpeak
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Old 11-25-2022, 10:13 PM   #8
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My 2007 fully built E350 6.0L weighed 10K#. 315's. I ran 50 - 55 psi on all four for 15 years just fine
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Old 11-26-2022, 10:53 AM   #9
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For our 2019 Transit 350 HR tire pressure is based on the load (https://tirepressure.com/lt245-70r17-tire-pressure)
Tires: BF Goodrich KO2 LT245/70 R17 119/116S E at 80 max psi; maximum single tire load capacity of 3000 lbs @ 80 psi
CAT Weight (fully loaded) - Front axle 4020; Rear axle 4920; Total 8960
Tire pressure BFG recommendations per load:
Front 45psi 2010, Rear 60psi 2470
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Old 11-27-2022, 04:15 PM   #10
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The correct tire pressure is a function of the load on the tire. Weigh each axle at a truck stop and adjust pressure according to this chart - https://tirepressure.org/lt315-70r17
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