Tire pressure

[nf6x]

Ok, dumb question time!

My -10 PDF says the proper tires for my favorite fresh-from-GL M1009 CUCV are "10 R15LT". That looks like an OCR error, and the CUCV FAQ thread agrees with the tires that are already on my truck: Uniroyal Laredo A/T 31x10.5R15LT, load range C, max load 2250 pounds at 50 PSI cold.

My -10 also says that the proper inflation pressure is 35 PSI, but my truck came from GL with "TP 45" stenciled over each wheel well. So, should I trust the TM and re-paint the markings, or should I run them at 45 PSI? Once I get it registered the truck will be driven mainly on roads and the freeway, probably unloaded most of the time, with just light off-roading since I live on a dirt road in a rural area. I have no plans to lift the truck or run big tires on it.

I have lots of pictures in another thread, but here's one anyway:

 

[JAsher45]

31 X 10.5 15's are the right tire for the truck Go by the tire manufactures reccomended tire pressure .... different tires different pressures... my good year wranglers are 50 psi tire and thats what i run on the road but if i am in the mudd kick them dowm a lil for more flex about 35-40

 

[nf6x]

Uh, I thought that the pressure marked on the tire sidewall was the maximum safe pressure at max load, not the recommended pressure at lighter loads. I'd think that if I ran them at 50 PSI at a load well under 2250 pounds each, then I'd get uneven tire wear.

 

[CCATLETT1984]

run what the tire says.

stock tires for the 1009 i think are 235 75R15 (something close to that, they are about 28" tall)

 

[AJMBLAZER]

Ummm...he's right, the number on the side of the tire is just for the max load capacity of that tire. Unless you're planning on running around with the Blazer weighing easily twice it's curb weight 50psi is FAR to high.
I can run up to 50psi in my 11.00R16 Michelins but that would mean the four tires would be capable of supporting over 18,000lbs! I run them at 25 in the front and 22 in the rear.

If I remember right GM had 30-35psi in the 31x10.5R15's in the 80's. If you want I could get the factory answer for you fairly easily over on www.ColoradoK5.com .

10R15LT is just an old tire size. It equates closely to 31x10.5R15. My 1986 civy owners manual has a bunch of sizes in it like that - 7.50R16, 6.50-16, etc.


The best way to do this is to either figure out what GM says and play with those numbers or to do the old chalk test on the tires.

 

[AJMBLAZER]

For a '91 K5 with 31's from the factory GM said 30psi in the front and 35psi in the rear. Fairly sure that is a 350 gasser truck and I'd bet the civy standard 15x7 wheels. Irregardless yours would be similar.

 

[nf6x]

I'm not familiar with the chalk test. I'm guessing that it involves marking the tread pattern with some chalk to see how quickly the center of the tread wears compared to the shoulders?

 

[JAsher45]

i know this may seem out of place but isn't this the same problem that was the issue of the Ford and Firestone tire issue... one said one pressure the other said a different and a few accidents later bam mass recall on tires.. in all the tires where not to blame but improper inflation....

 

[nf6x]

What AJMBLAZER said makes sense to me regarding the pressure marked on the side of the tires. Proper inflation pressure for on-road use would naturally vary with load. I think that the proper tire inflation pressure would be whatever it takes (within the limit marked on the tire) to get even pressure across with width of the contact patch, so that the tires will wear evenly between the center of the tread and the shoulders. My truck won't have anywhere near 2250 pounds on each tire, so 50 PSI would be too much.

I was just wondering whether there was some particular reason why my truck has "TP 45" stenciled over the wheel wells instead of the 35 PSI indicated by my -10 TM. Like, maybe the TM listed a smaller pressure as a compromise between on-road and off-road handling, while 45 PSI would be better for highway driving? The stenciled tire pressure on my truck could be a simple error, or based on faulty information.

I don't think that JAsher45's comment on the Ford/Firestone thing is out of place. Proper inflation is important for safety, and having either too much or too little can cause a tire failure. The manufacturer's recommendation isn't necessarily the best pressure to use; I've had multiple folks tell me that (some?) manufacturers routinely specify a lower pressure than is optimal for tire wear in order to provide a softer ride. So, without knowing how the manufacturer, the guy who stenciled my truck, and the folks who wrote the TM came up with their numbers, maybe I should play with my tire pressure to get the most even wear. Either by putting on lots of miles and watching the tread wear, or by doing something like the chalk test to figure it out more quickly.

 

[11Echo]

For more than 25 years I have been running 32/11.50-15 and 33/12.50-15 tires on a pair of GM K 10's. On the pickup, which for most of the time it is unladen, I run 35 front and 31 rear. This gave a good ride and even tread wear. If you run around empty most of the time, over- inflation of the rear tires just wears them thin in the middle of the tread. I pump them up to 45 for days where I am hauling materials/ firewood/ pulling a trailer, and other. On the K10 Suburban I ran 35 front and rear. It had more rearward weight with the fuel tank behind the rear axle and those 3 large pieces of glass. All my tires wore well running these pressures. This is not a suggestion for tire pressures. It is my tire pressure settings. YMMV.

 

[nf6x]

I think I'll trust the TM's 35PSI recommendation until/unless I determine that some different pressure would be better for my driving and loads, then. Ok, where did I put those 1" brass stencils...

 

[AJMBLAZER]

I think some non-thinking private stenciled the M1008 markings on there. I think 45psi went in the loaded down 235/85R16 Load Range E tires most of them and the 1028's had.

The Ford/Firestone issue was as you guys said. Firestone had some tires that were iffy in quality coming out of that Venezuelan plant. These unfortunately ended up on Explorers that Ford had decided the 35psi the tires need was too high and made them ride stiff. They lowered the pressure to 25psi and then these problems happened. Unfortunately a lot of bad info came out and Explorers (aka Exploders) and Ford both got bad names because of it.

Yep, the chalk test is putting a large patch of chalk across the tread and then on a long, flat, paved surface driving the truck back and forth and raising and lowering the air pressure in each tire until you uniformly wear the chalk off after driving in a straight line. You want the max pressure for this so it's best to start high and go lower.

 

[CCATLETT1984]

My load range E tires took 80psi for max weight.

 

[Fred431]

tire pressure

MY M1009 came with 40 pounds stencilled over each wheel

Fred

 

[jj]

RE: tire pressure

My 1009 has TP-50 stencilled over each tire. I've been running 50psi since i bought it in DEC. 2006. After reading this thread i dropped the pressure to 40psi. WOW! What a difference! The highway ride at 60mph was very noticeably less "harsh". Greatly reduced bounce on the frost heaves and much less of the tinny "boom" noise inside. Next trip i think i'm going down to 35 or 36psi. Oh, yeah, Goodyear Wrangler AT's: 31x10.5-15's on 15"x8" steel civilian wheels. The original G.I. issue wheels are wearing through on the lug nut bosses.

 

[hobie237]

RE: tire pressure

I too have wondered where the TP 45 on the fronts came from. It doesn't seem to match up to anything.

A few statements:

-The maximum safe pressure on the sidewall is just that- maximum safe pressure. No more, no less. It has nothing to do with what pressure you should run any other time.

-The Explorer issue was a bad combination of bad quality control, a poor engineering decision to drop the pressures, the fact that people don't check their tires regularly and they wind up underinflated, and the fact that people don't know how to react when they have a tire go down suddenly at highway speed

-The pressure you should run is indicated on the door sticker of any vehicle. Not sure if the CUCVs have the door stickers (haven't checked mine) but it should be an easy enough bit of information to glean from a civilian K30

-Tires should be given more pressure to handle higher loads. This was a problem in the Explorer case- the lowered recommended pressure may have been fine for when one person was driving an unloaded car down the highway, but stick four obese adults, a trunkful of crap and a full tank of gas in the car, and all of a sudden the tires are underinflated.

-Overinflated tires will wear unevenly and ride harsh, and provide less than ideal grip, due to the fact that the tread isn't making even contact with the ground, so you shouldn't just run around with the maximum pressure all the time.

-Underinflated tires are flat out dangerous and stupid. Not only will they provide suboptimal grip, and premature wear, and an inability to make sharp movements if necessary to avoid an accident, and reduce fuel economy, but they are prone to overheating and blowouts as seen in the Explorer case.

-When in doubt, err to the side of overinflation.