Tire pressure? 31x10.5 x R15LT F150 supercab

[oilboy123]

What would be a good tire pressure range for this non stock tire? The 31x10.5R15LT has max 50 psi on sidewall. I was thinking 40psi frt and 45psi rear. I know there are variables loads trailers etc. He does not tow and rarely loads it to capacity.
I'ts my brothers truck. F150 super cab 4x4, it shows P235/75R15XL as stock. Stock tire pressure range Frt 40 PSI Rear 45 PSI GVWR6250 LBS

Thanks in advance for opinions.....

 

[CapriRacer]

On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

If you have a different tire size than is listed on the placard, then the pressure should be recalculated based on the original load carrying capacity as expressed by the placard tire size and pressure. The calculation isn't difficult, but it requires the use of tire load tables - which are not allowed to be published on the 'net.

But I happen to have a copy and can do the calculation for you, but I am going to need 3 tidbits of information

1) The placard tire size

2) The placard tire pressure

3) The new tire size.

Well ....... we have all 3! Cool!!

So a P235/75R15 XL inflated to 40 psi front and 45 psi rear .....

OK, first problem. I know this is wrong. An Extra Load tire would not be specified at 45 psi by Ford. And when I look up Ford pickups in Tire Guides, it tells me that 15" tires haven't been supplied to F-150's since 1996, and they list ALL Ford pickups with this tire as using 35 psi front and 41 psi rear. While I'm not 100% convinced this is true, it isn't unreasonable, so I'm going to use those values.

Oilboy - Look for the placard (Should be on the driver's doorpost down near the door sill) and see if that information is correct. If it isn't, then the rest of this is also incorrect.

To get the same load carrying capacity, then a 31X10.50R15LT needs to use: 37 psi front / 41 psi rear

 

[Cup of Joe]

My 93 F150 comes stock with 265x75R15 same size as 31X10.5R15 which I'm running now. My plackard says 35 all round unless carrying a load of course. I find 35 front and 30 rear rides better with less rear end hop when empty. My truck is the lighter single cab 4x4 long bed model so 32-35 rear will probably ride better for a supercab.

I wouldn't run high rear pressure empty unless you like the excitement of going to fast on bumpy off ramps in the rain.

 

[alreadygone]

Capri's absolutely right on paper.

tater&nood's ablolutely right in real world.

Which one you gonna drive in?

Bob

 

[CapriRacer]

Originally Posted By: TaterandNoodles
My 93 F150 comes stock with 265x75R15 same size as 31X10.5R15 which I'm running now. My placard says 35 all round unless carrying a load of course. I find 35 front and 30 rear rides better with less rear end hop when empty. My truck is the lighter single cab 4x4 long bed model so 32-35 rear will probably ride better for a supercab.

I wouldn't run high rear pressure empty unless you like the excitement of going to fast on bumpy off ramps in the rain.


Unfortunately, there's a lot of misinformation in this posting.

According to Tire Guides, a 1993 Ford F-150 came either with P215/75R15's at 35 psi front and rear or P235/75R15 XL's inflated to 35 front / 41 rear - both on 6" rims. A P265/75R15 never came on the 1993 F-150 AND it requires a minimum of a 7" rim.

Further, the load carrying capacity of a 31X10.50R15LT is not the same as a P265/75R15 - in fact it is closer to a P235/75R15 XL - and for a while Ford used those 2 interchangeably.

But taking this a step further: A P235/75R15 at 35 / 41 psi has the same load carrying capacity of a P265/75R15 at .... wait for it .... 24 and 27 psi - which is actually lower than what Tater reported as what he uses to get a better ride.

Tater - you need to look again at the vehicle placard!!

The problem with listening to folks who don't know much about theory, but know what works for them, is that you have to make sure their information is absolutely correct - and that is usually a challenge!

 

[Cup of Joe]

Capri I have looked at the placard many times, the truck came with 265's and it says 35 all round. I can double check it this evening in case my age is starting to affect the mind.

I did just go through this last year when I bought new tires. My original firestone spare tire is a 265 as well. If I run 235's the speedo reads 5+mph faster then I am traveling.

I hauled 800 lbs about 2 weeks ago with 35 psi and over 1200 lbs with 40 psi rear. Actually I didn't adjust the psi for the 1st half of the 1200lbs trip then I stopped and add 5 psi which helped. I've also towed in excess of 5000 lbs with 35 psi, not sure of the tongue weight.

 

[Cup of Joe]

Discount lists 265 as OEM size as well. Tirerack did last year but not now for some reason.

F150 265's

 

[oilboy123]

Sorry it's 1992 year with aluminum rims 15x 7.5 wide.
The 15/75R15XL is correct on the decal, stock psi 35 frt & 41 rear.

I am thinking close to 40 front and 45 psi rear maybe that is too high Or maybe 35 frt and 41psi rear?

Am I in the ballpark? Thanks for the info guys.

 

[Cup of Joe]

I have to correct myself it says 30 psi front and rear 265/75r15. I have the same 15x7.5 rims, 30 psi is to mushy for the front but great for the rear which is naturally lighter.

My stock placard says 30 empty, you won't like 40-45 if your empty. The most I have run with a load is 40 psi. The ride will get pretty hoppie if you run in the 40's empty.

 

[oilboy123]

I think my brother mentioned the ride being choppy at 40 psi frt 45 psi Rear. Maybe 35 front 41 rear will be OK with the aftermarket tires?

 

[CapriRacer]

Originally Posted By: TaterandNoodles
I have to correct myself it says 30 psi front and rear 265/75r15. I have the same 15x7.5 rims, 30 psi is to mushy for the front but great for the rear which is naturally lighter.

My stock placard says 30 empty, you won't like 40-45 if your empty. The most I have run with a load is 40 psi. The ride will get pretty hoppie if you run in the 40's empty.


Interesting!!

2 possibilities:

1) The book is wrong (wouldn't be the first time!)

2) That the tires were not applied at the Ford factory, but were applied at a converter - and that should mean the placard is a non-Ford placard. Easy to check this out. It ought to be obvious when a placard is not an OE placard. It will not have all the factory stuff - like a Ford Logo.

Tater, I hate to ask you to look again (us old guys don't like bending over), but could you verify if the placard is a Ford or not? That way I can handwrite a correction into my book.

And while you are at it - does it list both a loaded and empty pressure (and other assorted information)? A photo would be nice!

 

[ARB1977]

Man try riding 26 front 29 rear...way too soft for my liking, even on a lighter truck. I run 33 rear and 34 front on 265/75R16's.

 

[Cup of Joe]

Originally Posted By: CapriRacer

Interesting!!

2 possibilities:

1) The book is wrong (wouldn't be the first time!)

2) That the tires were not applied at the Ford factory, but were applied at a converter - and that should mean the placard is a non-Ford placard. Easy to check this out. It ought to be obvious when a placard is not an OE placard. It will not have all the factory stuff - like a Ford Logo.

Tater, I hate to ask you to look again (us old guys don't like bending over), but could you verify if the placard is a Ford or not? That way I can handwrite a correction into my book.

And while you are at it - does it list both a loaded and empty pressure (and other assorted information)? A photo would be nice!


I took a pic but for some reason it will not load to our pic hosting site, none of the pics I loaded on the wifes PC this morning are loading to picturetrail. I blame it on windows Mohave.

I should be able to email it between bites of stuffing.
Happy turkey day.

 

[Cup of Joe]

Well after turkey day and spend all your money day we have finally found time to figure out the problem and get the pic posted.

Ah Cr*p hang on.

 

[Cup of Joe]

Try it again.

 

[CapriRacer]

Tater,

I stand in awe! Good job. And thanks.

Yup, this looks like a factory original.

Oh, and since we have this up, notice a few things:

A P265/75R15 has a load carrying capacity of 2309 @ 30 psi. Since this is a light truck, the load carrying capacity has to be derated 10%, so we divide by 1.1 to get 2099#.

Compare that to the GAWR's (Gross Axle Weight Rating): 2950# front / 3776# rear. That means the tire pressure is specified such that the tire has more capacity than its intended usage.

This is one of the lessons fron the Ford / Firestone situation. Over-specifying tires results in fewer failures. Of course, that princple works in every situation - and not just tires. It's just that it was not unusual to find the tire capacity and the GAWR to be the same.

Also notice that the GVW (Gross Vehicle Weight) is 6250# where the GAWR's add up to 6726#. This is to account for assymetrical loading. The point being that the load might not be evenly distributed - and the vehicle manufacturer is accounting for this.

Side note: This general principle of over-specifying also applies to speed ratings - so I cringe when folks say they don't need an H speed rated tire (or whatever) if they aren't driving 120 mph. Not to mention that a tire's speed capability changes with inflation pressure, ambient temperature, and age.

 

[tom slick]

CapriRacer, what is your opinion on installing LT tires on a F-150 that originally spec'd P tires?

 

[CapriRacer]

Originally Posted By: tom slick
CapriRacer, what is your opinion on installing LT tires on a F-150 that originally spec'd P tires?


I am not a fan.

Normally, this change involves increasing the inflation pressure in order to keep the same load carrying capacity. This doesn't sound like a good idea for the truck.

 

[MNgopher]

I once went from a P rated tire to an LT rated tire on my '99 F150. At the time, it was a good move. Going through the load carrying capacity calculations, I did end up running a higher pressure in the LT tires than the P series tires. I prefer a "firm" ride, and the new LT series tires took away some of the waffling I felt on the P series tires. I had been planning on replacing my shocks a at that point and was able to go on without the shock replacement.

Since that time, I've gone back to a P-series tire (although 3 steps up in section width) due to cost and availibility issues (nobody had a Load Range "C" tire in stick or remotely close to in stock)...

 

[vance1]

So let me get this straight. Some people actually have a different size tire than what the placard states. I have just been severly chastised over in the maintinence forum for putting a truck on the road that I just bought that the placard calls for tires with 41 psi and the tires that are on it call for 35 max psi. It is a 2000 Dodge Ram 1500 4x4. The placard states I think p225/75r16 at 41psi and the truck has Toyo Open Country A/T 265/75r16 35 max psi. So who is right or am I just an idiot? Do not answer the last part of that question. My post is in the maintinence forum under very dangerous tire bounce.