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Old 10-12-2012, 05:24 PM   #1
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Under standing Truck Tires

What does P255/70R18 mean? Understanding what you have on your Pickup truck is not necessarly what is suppose to be on your truck. My 2008 Tundra SR5 I-force, that I bought (used) from a dealer had P255/70R18 on it. I'm not sure why the red flag didn't raise when I looked at the spare tire mounted on a steel rim. I guess I was impressed by still having the nubs on the tire. I knew the other steel wheels were exchanged with aluminum wheels (they do look better), but never thought the tires were not the size/rating that the manufactor installed. Last week I started getting a false low tire pressure indicator (verified all at 32psi), took it to Toyota and found that P275/65R18 are the correct size. The tires look like this due to hauling my 5th wheel (2011 Flagstaff 8528BHSS). Seams P255/70R18 can't take the weight/flex. I'm including a link to better understand tires for all to use.
www.rimsntires.com/specs.jsp Never enough said about "Understanding Truck Tires"

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Old 10-12-2012, 07:01 PM   #2
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P is for passenger car tires you need to get LT tires and try for E rated thats a 10 ply tire
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Old 10-12-2012, 07:41 PM   #3
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I'd also recommend running LT tires load range E.

As you mentioned verifying that the pressure in the P metric tires are 32, that may also be some of the problems you're seeing. The max pressure for that tire is 44 psi and that's what you should be running whenever you're towing.

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Old 10-12-2012, 09:46 PM   #4
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As states before the P stands for passenger the 255 is 255 millimeters across the tread and the 70 is 70% of 255 millimeters which is the side wall height the R stands for radial and the 17 is the rim size. You can have many different number combos and the tire will be the same height.

For example a 285/75r16, 285/70r17, 285/65r18, 285/60r19 and 285/55r20 are all exactly the same height. When you go to a bigger rim size the sidewall gets smaller which is where you see the second number smaller in each size.
A 305/70r17 is also the same height but is wider.

And your rims were probably not exchanged out for aluminum the spare is often steel and not matching. I know on the tundras that is how it is. And by the looks of those tires it's time for new ones. And definately go with a LT tire and I would go with at least a load range D which would be an 8 ply tire. In that size I am not sure they would make a load range E 10 ply tire unless you went to a different size which is possible also.
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Old 10-12-2012, 11:07 PM   #5
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Yep what he said. I work ar a chevy, buick, gmc, toyota, scion dealership. I have never seen a truck that did not have a steel spare wheel. On a very rare occasion I will find an aluminum spare for cars- usually camarys and mercedes.

As stated above tire size doesn't matter as to the load rating. If it doesn't have a load rating such as C-(6 ply), D (8 ply), E (10 ply) listed on the sidewall it will be a 4 ply passenger tire.

You may check the Dot date on the tire, as tires have a expected life of 5 years ( seems like less though). This date will be found on the side wall in a oval with a 4 digit date such as 3512 which would be the 35th week if 2012 there may be a few letters or "dot" in front of the numbers in the oval- kind if differs between manuf. Kind of looks like yours have aged out but my dw car had a tire that had some dry cracking at about 2 years old.

P.S. I don't think that your 255 really was that much different in actual size than the 275, but you can check yourself if you search " tire size calculator".
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Old 10-13-2012, 11:06 AM   #6
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Is there a web site that explains in detail the proper tire pressure for loads? If I know the load weight how do I know the proper tire pressure?
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Old 10-13-2012, 12:13 PM   #7
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If you ask a 100 people you will probably get 100 answers, but I will give my opinion. The safe answer from a liability standpoint is to follow what you door jam sticker says. Although from ford and firestones history this is sometimes not a good answer and they were held liable for damages.

On pass tires (max 44psi) I am not sure but on the dw tahoe I try to keep around 40psi.

On C rated tires (6 ply 50psi cold max) it is best to keep at max.
On D rated tired (8 ply 65 psi cold max) it is also best at max
On E rated tires (10 ply 80psi cold max) they are intended to run at 80psi.

The load ratings are at max rated psi. If you go down much in pressure your sidewalls will flex too much and create heat which negitively efect the tire life. If you go above the COLD rating when cold (not warmed from travel) then you will possibly have a true blowout. I am not sure what pressures pass(4 ply) tires are rated at so I can't tell you on them.

If you run a high load rated tire on a lighter vehicle you can air down some. On heavier trucks it is recomended by manuf. To run less psi on front end sometimes and tire ride and stiffness may having you want to air down to keep from getting bounced around as much.
Ps. I don't know about a website but I'm sure there is one out there. Discount tire tells you inflation on your reciept when you buy tires.
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Old 10-13-2012, 12:51 PM   #8
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Lots of great feedback so far. Now for the 2nd question. How safe would it be to run P255/70R18 on the front with P275/65R18 on the rear? I mean with the bulk of the weight on the 5th wheel hitch. I do not have 4 wheel drive on my Tundra so I don't think that would be an issue.
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Old 10-13-2012, 01:00 PM   #9
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Don't do it all the brakes have speed sensors on them that work for the abs and traction control. If you do that the front and rear will spin at different speeds and will cause you problems. I know there's a LT 275/65/r18 that is in a load range c 6 ply and probably have a load range d 8 ply. Go with that.
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Old 10-13-2012, 01:05 PM   #10
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Just ran your # on the tire calculator at 1010tires.com and this is what it came up with

255-70-18 is--- 10.3" wide & 32.05" tall

275-64-18 is ---10.82" wide & 32.07" tall

The width is about 1/2" wider on the 275s and only .02"taller so yes you would be fine running them like you stated!!
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