Why do my 31 inch tires measure 29 inches. - Page 2 - Ranger-Forums - The Ultimate Ford Ranger Resource


Wheels & Tires Semi-Tech General discussion of wheels and tires for the Ford Ranger.
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  #26  
Old 06-06-2006
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Quote:
Originally Posted by optikal illushun
umm...ur not running stock sizes tires (and neither is a lot of people, not isngling u out) so throw the manufactors spec out the window...BUT running them at max spec on the sidewall is silly. plus sidewall stiffness is a factor, 285s are a class d? tire (most are) and require more pressure than a class c? (normal tire)...

wanna know the best way to get good contact patch? mark 6" of ur tire on the tread with chalk and fill up to a desired PSI. then move the truck so the chalk leaves a mrk on ground and u can see how the contact patch is...adjust is nesseccary...

I concur.
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  #27  
Old 06-06-2006
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isnt running tire that high also very bad for the tire, because the trucks recomedation is the best thing to go by because its compting the weight of the truck in with the pressure...... i think 65 psi isnt running pressure, its jsut what the tire can handle till it bubbles...


check what your PSI is sean after a 15 mile trip in 80+ weather on the highway, its probably much higher....
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  #28  
Old 06-06-2006
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My PSI is at 52hot(after driving the 240mile trip) and 48cold (and i have checked that many many times)

normal driving puts me right at 50
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  #29  
Old 06-06-2006
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When I go offroading at Baumhart, I air down to 15. I've tested it, haven't gone lower but I do a lot better through the trails at 15psi then 20 or 25, which is what i had intially aired down to . Then i refill up to 35psi, and never check it.
Sometimes i think i should air up alot more, but too lazy.
I can't believe some of the air pressures you guys are running, i've always been told to go inbetween door jamb and tire reccomendation
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  #30  
Old 06-06-2006
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Quote:
Originally Posted by optikal illushun
umm...ur not running stock sizes tires (and neither is a lot of people, not isngling u out) so throw the manufactors spec out the window...BUT running them at max spec on the sidewall is silly. plus sidewall stiffness is a factor, 285s are a class d? tire (most are) and require more pressure than a class c? (normal tire)...

wanna know the best way to get good contact patch? mark 6" of ur tire on the tread with chalk and fill up to a desired PSI. then move the truck so the chalk leaves a mrk on ground and u can see how the contact patch is...adjust is nesseccary...

Out of all the replies, this one is the closest to the truth. Wow, the way you guys set your pressures is scary. Here is the bottom line of this whole topic.

Lets look at the stock tire size of a truck, we'll take my Edge for example, a P235/75R15. Ford specifies 35 PSI for this tire on this truck. Now if we look at a set of Load/Inflation tables you will see that this combination of tire size & pressure has a load capacity of 2028 lbs per tire. This is the standard that the replacement tire must have.

Now flipping over to the Light Truck Load/Inflation tables we see that to achieve 2028 lbs of load capacity with the 31X10.5R15 that I have on my truck, it requires 48 PSI to do this. This is what you need to inflate your tires to.

This is the method a knowledgeable tire facility will use to determine the correct inflation pressure....no guessing, or speculation here.

If there were a means of posting the inflation tables I've referred to on the forum I would put them up so that you guys can see what is involved. Its really a relatively simple process if you have access to the right info.
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  #31  
Old 06-07-2006
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Quote:
Originally Posted by auto_god
Lets look at the stock tire size of a truck, we'll take my Edge for example, a P235/75R15. Ford specifies 35 PSI for this tire on this truck. Now if we look at a set of Load/Inflation tables you will see that this combination of tire size & pressure has a load capacity of 2028 lbs per tire. This is the standard that the replacement tire must have.

Now flipping over to the Light Truck Load/Inflation tables we see that to achieve 2028 lbs of load capacity with the 31X10.5R15 that I have on my truck, it requires 48 PSI to do this. This is what you need to inflate your tires to.
That might be true if your loaded Ranger weighed 8112 lb with equal weight distribution (2028 lb) on each tire.

It just isn't that simple. The recommended tire pressure is determined from a combination of considerations. True, load capacity is important but so are ride quality, handling, wear, temperature, etc. A vehicle manufacturer takes all of these into account to develop the pressure recommendation on the door jamb label. The recommended tire pressure is always a compromise.

When Ford determined the recommended pressure for your P235/75R15 tires to be 35 PSI, it's fairly obvious that they were not doing it based on anticipated tire load alone.

A Ranger loaded to Ford's recommendations has a maximum Gross Vehicle Weight Rating of only 5440 lbs (or less). If you take half of the 8112 lb figure, that's 4056 lb for each pair of tires. There's no way that either the front or rear tire pair is ever going to see anything close to 4056 if the max GVWR is 5440 (or less for most Rangers).

The Level II Ranger comes from the factory with 31X10.5R15 Load Range C tires and a 5440 GVWR. The recommended pressure is 30 PSI. If someone chose to install Load Range D or E tires in that size, the optimized tire pressure might be slightly different but it would require an engineering group and a test facility to determine it.

There is absolutely no need to run 48 PSI in a 31X10.5R15 under a Ranger.
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  #32  
Old 06-07-2006
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You are correct on several points....& I'll clarify a few of the things I stated in my previous post.

8112 lbs....yeah, no Ranger is ever going to see that. Really, the 2028 lb number is arbitrary...it could be 5000 lbs per tire, but the point of my post is that the replacement tire & inflation pressure must be capable of meeting the specification of the OE size/pressure. After the Firestone/Explorer recall, no tire manufacturer or dealer would recommend anything less.

the optimized tire pressure might be slightly different but it would require an engineering group and a test facility to determine it.


This is correct also. However, everyone on this forum doesn't have access to this, so it would be far more prudent to follow the Tire Industry Association's accepted guidelines for determining substitute inflations than to just throw your best guess-timate out there. The ramifications of a tire failure due to under-inflation can be serious, even potentially lethal if it were to occur at the wrong time. Thats an awful thing to have on your consience. So without the testing facilities & such, the only tool we have to work with is the OE load capacity.

There is absolutely no need to run 48 PSI in a 31X10.5R15 under a Ranger

I am running 48 PSI on the fronts on my Ranger & the wear has been perfect & the driving characteristics mirror the originals in terms of steering response, high speed stability etc. I do run slightly less in the rears at 42 psi, but from a professional standpoint being in the tire business I would never advise anybody to run a pressure that doesn't meet the OE load capacity....thats just a liability nightmare waiting to happen. Its always better to err on the safe side.

I'm amazed that forum after forum that I read through, people continually make pressure recommendations based simply on "this is what I run" rather than on Tire Industry Association approved guidelines. I don't want to be the guy who gets somebody hurt because my recommendation was flawed.
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  #33  
Old 06-07-2006
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I make no recommendations. The original manufacturer's rating for the original tires on any particular vehicle is all that's available. When someone chooses to modify their vehicle with another size tire, tread pattern, ply construction or orientation, load rating, etc., they change the equation. Clearly, the average consumer does not have access to the kind of testing required to reevaluate the pressure for this kind of modification.

My point is that your method of choosing tire pressure by matching load across two tires is not valid when the original tire pressure was clearly not chosen based on load.

The only information I have for a 31x10.50R15 on a Ranger is Ford's recommendation of 30PSI on a Level II. This inflation spec is only for a BFG A/T Load Range C on that particular model. Installing these BFG's on another model of Ranger or using other tires or sizes or load ranges may change the optimal pressure compromise.

Sure, underflation is dangerous and any error should be on the side of caution. However, overinflation has its negatives as well. 48 PSI is a long way from 30 PSI.
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  #34  
Old 06-07-2006
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Bob is right!!! Damm i am getting sick and tired of this stupid topic. Do a search and you will see that i have talked about it many time before, but again for those who need it.

For normal driving you do NOT!!! need to run MAX PSI.

Our trucks ranger from 30/30, 30/35, to 35/35 for the stock pressure. when your tires are stock you should run what the door jam/owners manual says.

Now when you start switching tire sizes you are more then likely going to have to play with your air pressure. Load range will play a role, but generally speaking you are probably going to drop air pressure a little with a bigger tire. I learned this the hard way when i ran 30/30 with my 31's then started to get the centers wearing out, now i run 28/28. be careful not to lower the pressure too much, especially below what is recommended.

For all of you guys going off the sidewall, stop it unless you are fully loading your truck down, and even then are rangers are not going to get loaded down enough for max psi. out tires are also good for full-size trucks that when fully loaded might use the max psi on the sidewall(mainly talking about our 31'+ that are LT tires). This whole crap about the sidewall says ___ so that is what the tire is designed to use is stupid. I can't imagine anybody on this site needing to run more then 35, or just maybe a little more, on a regular basis!

Sean, first off read what i have above, but i have something extra for you ( )and everybody else. you mention when the tire is hot that it says 52psi and 48 when cold. a tire is considered hot after a couple of miles(its been awhile since i took the test, i think it is 2 miles). it takes about 4-6 hours too cool down. so your normal driving would be under the 52 not the 50psi. our policy at work, that is dictated by Michelin, is what i just stated, hell they are the tire guys they know more then any of us on this site! when your tires are hot you are supposed to add 4psi if you are adding air. still the best time to check your air is when you haven't been driving for a long time.
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  #35  
Old 06-07-2006
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I've upped my pressure to 35psi front and back on my 31x10.5R15 BFG AT equipped 2003 FX4 LII. At ~37k miles my tire wear has been good. Handling and traction have been excellent.
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  #36  
Old 06-07-2006
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Jacob (and everyone else)- When i am home and am just doing short city driving my tires are at 45 PSI, but during thr school year (when all my driving is the occasional trip home and the truck just sits in the school parking lot) i have them at 50 for the maximum gas mileage. The only messed up wear i have on them is when my front end was ****ed up and i wore the insides of the tires a bit more then the outside (it has since been fixed). I guess my question then is that if it is a 65 psi max, 45psi is still 20psi under that (1/3 of the entire tire pressure). I just cant see airing way down to 30-35 for normal driving and getting even close to the same MPG and such... Plus with the tires deflated more wouldn’t the wear be faster then inflated(within reason)
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  #37  
Old 06-07-2006
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Your tires are going to last you the longest when the tires has the most tread surface in contact with the ground. when you over-inflate you are causing the centers to wear out quicker and leaving tread on the outside not being used. an over-inflated tire would do better for gas mileage, but considering the miles you loose on the tire i would say that it won't pay off in the end.

in the end though the perfect pressure is going to be what ever gives you an even tread wear. for me that is 28/28. either way i find it hard to believe that 40+ is going to give some of you even tread wear.

p.s. sean, i didn't mean to point you out so much as just address the hot air pressure vs. cold, that you mentioned.
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  #38  
Old 06-07-2006
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hahaha its ok Jacob, it would be hard not to point me out. mmm when i head home for summer Friday afternoon i'll give my tires a good look-over... plus they are currently at 40psi after my last off-road trip so i think i'll drive home on that and see my MPG
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  #39  
Old 06-07-2006
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This site's about airing down for off-road and stuff, but there's a graph I think you all should look at.... this graph was based on BFG a/t's 30"x9.50" r15...

http://4wheeldrive.about.com/gi/dyna...%2Fsfjun96.htm
Look at how shallow the shope is above 30 psi!!! You people running above like 40+ have GOT to be kidding me... for you that air down... look how far down you have to go to make a difference in footprint size... say you normally run at like 45 (50 for Sean, haha)... you're sittin' there and you bring 'em down to 30... 15 psi difference (20 for Sean)and you say to yourself "wow... I aired down a LOT"... not really... look at how high you sit on that curve...

What do I run?... 34 front/30 rear...
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  #40  
Old 06-07-2006
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Quote:
Originally Posted by Oasis
This site's about airing down for off-road and stuff, but there's a graph I think you all should look at.... this graph was based on BFG a/t's 30"x9.50" r15...

http://4wheeldrive.about.com/gi/dyna...%2Fsfjun96.htm
Look at how shallow the shope is above 30 psi!!! You people running above like 40+ have GOT to be kidding me... for you that air down... look how far down you have to go to make a difference in footprint size... say you normally run at like 45 (50 for Sean, haha)... you're sittin' there and you bring 'em down to 30... 15 psi difference (20 for Sean)and you say to yourself "wow... I aired down a LOT"... not really... look at how high you sit on that curve...

What do I run?... 34 front/30 rear...
nice find nick!
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  #41  
Old 06-07-2006
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thats a sweet graph nick.
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  #42  
Old 06-07-2006
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bout time someone found a graph or something I gave up arguing
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  #43  
Old 06-07-2006
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aww.... you guys... stop it....
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  #44  
Old 06-07-2006
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Wow with all of this new info, I must appologize to Anthony for my grumpyness yesterday and errogance. You were right and I was wrong.

On that not, with a LT285/75r16 what psi should I be running?35? I ask this only because the graph only talks about a 31x10.5x15 .
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  #45  
Old 06-07-2006
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Im not really sure but I would run 30 to 35 on a bigger truck like yours, I wanted proof too I was just always told run mfr specs in any tire by the michelin guidelines. I am a service mgr at wal-mart thats how we always done it but I quit arguing cause i coulda been wrong too. IF the graph is right I would say 35.thats what I run in mine.totally dif tire though.
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  #46  
Old 06-07-2006
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Keep in mind... the graph is for a 30"x9.5"r15 BFG a/t... although I suppose it could work with other sizes as well... there ways of finding out what you should run at on the link I posted and someone on here brought up "chalking the tread" which seems right too...

Let's put it this way... the guy who wrote the article and came up with that graph, runs.... "run 26 PSI, night and day, seven days a week, and typically get 60,000 to 80,000 miles out of a set of BFG T/As, including sand runs at much lower pressure."... 26 psi for a 30" tire and he's getting that kinda mileage...
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  #47  
Old 06-07-2006
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mmmk. ill try 35 see how it feels
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