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Old 01-12-2016, 04:35 PM
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Tire pressure question

I have the Discoverer STT Cooper 33x12.50R17LT tires on Cragar rims with 2.5" lift kit. What pressure should I run to optimize MPG while not making it feel too stiff of a ride.

I am currently at 35 PSI.

Thanks guys


2012 sport jk

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Old 01-13-2016, 05:02 AM   #2
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Depends on what Loadrange you have, search for that on sidewall.
Better would be if you find something like this, and I used the values I found with google.

C-load ( LRC) "maximum load 2040 lbs AT 50 psi ( cold)" wich is 105 Loadindex for single load.
D-load ( LRD) "maximum load 2600 lbs AT 65 psi ( cold)" wich is Loadindex 114 for single load.
Can be that for these sises sometimes C-load is 35 psi and D-load 50 psi.
And European system is a bit different.

They are LT , you already gave that.

Simple calculation is linear and is not even that bad, but I can calculate it with even better formula then the European tyremakers use, and sertainly better then the calculation of USA and canada.

And because the maximum load your axles are alowed to carry by law ( GAWR's stand for Gross Axle Weight Ratings) , is probably about the maximum load of one tire on that axle, you would expect about half of the AT-pressure ( wich is not the maximum allowed cold pressure of the tire).

So try to find these data and I will calculate.
Also max speed of tire and what you use and wont go over for even a minute.

Greatings from a Dutch Pigheaded Self-declared tirepressure-specialist.
Peter

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Old 01-13-2016, 10:10 AM
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Load Range D, Max Load 2600lb

here's the link to the tire:

Cooper Tire & Rubber Company - Discoverer S/T™
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Old 01-13-2016, 10:21 AM   #4
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Just experiment, and don't go over what the tire says. I recently did a comparison of exact same BFG tires, same diameter in a C load at 40 psi and E load @ 32 psi. It was an eye opener, and noticed the C's had more rolling resistance likely due to the softer sidewalls. I had to push the gas pedal down further to get it rolling. E's got much better mpg and the crazy thing, it rode smoother and the jeep was less jumpy. I really thought the lighter weight and softer C's would be better, because that's what many were telling me. Find out for yourself!
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Old 01-14-2016, 10:42 AM   #5
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Quote:
Originally Posted by branimal View Post
Load Range D, Max Load 2600lb

here's the link to the tire:

Cooper Tire & Rubber Company - Discoverer S/T™
Did not find maximum speed on tire , but can do without it , because your maximum speed wont be that high I think. Googled for the conversed to Eurometric tire 315/65R17 and gave in 8PR 118 and 6PR 113 loadindex , so I think the At-pressure of your D-loads is here 55 psi , but mayby you can read it from sidewall for me. there is still some daubt about that ,because C-load is 50 psi and 6PR is 55 psi and your Loadindex is 114 then AT 50 psi , while 6PR gives 113 Loadindex at 55 psi.

Now only have to know the weights on axles or at least the best possible estimation. For this you need empty weight and build of car ( motor in front I think for weightdivision) , and the way you load it ( persons and load).

for this you need to look at the tires sidewall, and the car or manual.
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Old 01-18-2016, 08:32 AM
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LOAD RANGE D.

max load 1180kg (2600lbs) at 350kPg (50 PSI).
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Old 01-19-2016, 02:53 PM   #7
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OK then , there still is a difference between the Eurometric tire and the sises given in inches and still your tire gives LI114 with 1180 kg maxload at 350kPa/51psi while eurometric 315/65R17 gives 113 LI with 1150 kg maxload at 380 kPa/55 psi.
12,5 inch is though 317.5 mm so this might give the higher maximum load .

So I will go from the last tire data you gave , and will make a pressure loadcapacity list for that with spreadsheet I made for that.
Given in kg and lbs, psi and kPa/barr and for single and dual load given per tire, so per single axle times do times 2 and dual axle do times 4 .

then you only have to determine the loads per axle or tire and look it back in the lists.
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Old 01-19-2016, 03:41 PM
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I must admit I don't quite follow. Can you tell me how much PSI I can safely run? I'm hoping to improve my fuel efficiency.
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Old 01-20-2016, 03:29 AM   #9
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answer to above.

For that you first have to determine what loads are on the seperate wheels , or second best the axles.
Once you determined them , look back in the list with pressure and weightkind you normaly use ( there are 4 for all the combinations) to see what pressure you need for that weigt.

I can help you with that to , but need empty weight of vehicle and loaddivision Front/Back ( but can estimate that too) , and the way you load it ( persons and load and about where you put it).

Also handy is Gross axle and Vehicle weight ratings ( GAWR, GVWR) wich are the maximum allowed total weight on axles /vehicle by law, and often are used to determine the advice pressures given on car somewhere.

In my lists I also give weight for higher pressure then AT-pressure , yust for to look back pressure if you add a 10% reserve to the determined loads.
The tiremaker doens not allow that more load then maximum load given, but does allow more pressure above AT-pressure.
The AT-pressure is not the maximum allowed cold pressure of a tire.
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Old 01-26-2016, 11:09 PM
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unloaded weight is 4250lbs.

i am usually the only passenger. 160lbs

bike on top: another 40 lbs.
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Old 01-27-2016, 04:54 AM   #11
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Then I do my loaddivision thing I once determined, but mind its still a estimation.

Unloaded weight of 4250 lbs .
This is because motor in front, 45% of Unloaded weight +300 lbs= 2212 lbs on front axle empty.
Rear 55% of UW-300= 2038 lbs

We add driver of 160 lbs wich gravity point is about in the midle between the axles for a Jeep ( Belly button is about the the place of the gravity point of a person sitting as you drive a car)


The same for the bike on top also about between the axles devided so adds about 20 lbs to each axle weight.

So when driving as you do about 95% of your use the axles weights are next.
Front 2312 lbs and Rear 2138 lbs.

Then we add 10% to that for reserve , for things like , pressureloss in time, incidental extra weigt, misreadings of pressure and misyudging of weight , and unequall loaddivision between the wheels on the axle,gives next weights to look back in the list or calculate the pressure for.
Front : 2312+10%=2544 lbs on the axle so 1272 lbs on each -fronttire.
Rear : 2138+10%=2351 lbs.on the axle so 1176 lbs on each rear-tire


This we look back in the list I made and it will give a pressure at wich still no bumping ( Yeeps-desease) and stil acceptable gripp , but no exesive heatproduction so save for the tires.
Mind that this only counts if I determined the loads on tires correct.

Now looking that back I see its below 27 psi , wich is lowest given in my list so have to calculate it for you or make a new list.
Because you probably dont dare to go lower , you are sertainly not to low with this 27 , but will use my motorhome calculator for it, and give answer in next post.

Sorry again for the long post.
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Old 01-27-2016, 06:48 AM   #12
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Did a calculation with my motorhomelighttruck calculator and set the reserves so it can be used here.
Gave next
Front 24 psi
Rear 22 psi
So even if you would use 24 psi all around you wont damage your tires when on road driving at maximum speed of tires. Even some reserve build in .
So feel free to experiment to lower pressure , as long as you dont go lower then this advice.

For fuel saving and road handling , you can go higher , because my determined border at wich bumping begins is discussable.
I state that comfort is still acceptable if real weight on tire is above 85% of weight the pressure is calculated for.
For Trailers I state that screws begin to turn loose. if real weight is below 80% of same story.

But mayby these borders are lower , and its subjective, what one experinces as discomfort, the other one wont have problem with.
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Old 01-27-2016, 08:59 AM
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thanks jadatis. i am currently running them at 35 psi b/c i want better fuel efficiency. getting like 10mpg right now. I will drop them to 27 area to lessen uneven wear on the tire.
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Old 01-27-2016, 03:03 PM   #14
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Realise that the fuel the tires together spoil is at speed 50m/80km/h about 10% of total .
If you pump up your tires harder , you can save about 5% .
once did som estimated calculation for it, and you better go , instead of Miles/galon to gallons/100miles.
then a sertain pressure so deflection is for every speed the same fuel consumption.
For instance 1 gallon/100 miles.

Roling resistence is lineair so 2 times as much speed gives that 2 times as much fuel needed for 100miles .
So 2 gallon/100 miles.

So in your case 10 miles/galon= 10 gallon/100 miles.
Lets asume 10% of that energy is gone by roling resistence.
Then 1 gallon/100 miles consumed by the tire-resistence.
If you save by higher pressure in tires half of that , it gets 0,5 gallon/100miles.
so total fuel consumption= 9.5 gallon /100miles.
this is 10.52 miles a gallon.
So by giving tires a higher pressure your M/G only goes down with 0.52.
Yudge for yourselfes if this is worth it for you.

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