I completely agree Mike. Fortunately we are talking about “bad” and my friend only using it as a guideline and in this case, putting in 40 lbs which is just 4 lbs over manual and jam info. My point is that many of us would consider it under extreme conditions including “bad” and my friend…and me too
The “bad” is the recommended pressure refers to the recommend (factory) tire size…
Again I have oversized wheels and tires.
I do understand that with heavy loads I want to run a higher pressure,
My question was more about the temperatures I drive in.-40 & -50
I don’t believe anybody hit on that.
COLD
My question was more about the temperatures I drive in.-40 & -50
Temp has NOTHING to do with what you want to set the tire pressure at. Temp will effect tire pressure in that for every 10 degrees your tire pressure drops 1lb. Example: If you inflate your tires to their recommended pressure of 32lbs when it’s 70 degrees outside…If the temp later that day reaches 30 degrees…the tire pressure will have dropped 4lbs. If the temps stay at 30 degrees…then you’ll want to add more air to the tires to bring the pressure up to the 32lb rating. I just had this problem this morning. The cold morning (34 degrees)…my tire pressure light is on. It got cold enough for my tire pressure had dropped a few lbs since I last checked it.
No matter what the outside temps are you still want to keep the tires inflated to their proper spec as much as you can.
Copy that.
Thanks
Joseph, with respect, if the size of my manual is the size of your phone book, you must live in a very, very small town. Owners’ manuals are small and well indexed. Owners should read them. Besides, the manufacturers IMHO go to extreme lengths by putting a claer label right on the doorjam to try to get people to know their tire pressure. People are just too lazy to bother lookiing. The information is all there.
Ya’ know. A bit of info would be cool.
What vehicle are we talking about? What was the original tire size? What’s the current size?
96 laredo
Factory: 215/75R15 - 36psi
Current: 245/75R16
Current tires are too big in my opinion
I’m thinking about going to 15" rims with 30x9.5 rubber.
Are 30x9.5 - 30 inches in diameter?
That is a substantial difference in tire size with 3 inches in diameter and a 10% difference over all.
In this case, I amend putting more then 36 lbs in and would put in LESS. IMO, you could easily be running 30 to 32 lbs. Personally, unless you have a problem with handling, I would keep the lrger tires and use lower pressures. Experiment a little. The plus sizes for you car don’t appear to be much different…at least in the newer models.
The 215/75R15 was the minimum tire size on the 1996 Grand Cherokee, 225/75R15 was much more common. The 235/75R15 was an option and 245/75R16 is about 1 1/2" larger in diameter.
Later years used the 245/75R16 (WJ) and the tire pressure label usually reads 33 PSI.
On a side note I found an interesting statement in the 1996 ZJ service manual;
Chrysler Corporation advocates driving at safe speeds within posted speed limits. Where speed limits allow the vehicle to be driven at high speeds, correct tire inflation pressure is very important. For speeds up to and including 75 mph (120 km/h), tires must be inflated to the pressures shown on the tire placard. For speeds in excess of 75 mph (120 km/h), tires must be inflated to the maximum pressure specified on the tire sidewall.
I’ve no doubt that that’s all correct, Nevada, however one needs to be aware of rim sizes when looking at optional tire sizes. Manufacturers will often offer different tire options, but usually on rims that are different widths.
16x8
There’s also the difference in tire type. The 215/75R15’'s were P type tires and the 245/75R16’s are LT type tires - different animals.
Doing the math, the LT245/75R16’s have more load carrying capacity at 35 psi than the P215/75R15’s do - and 35 psi is the lowest pressure i would recommend for LT type tires.
Hence my question; what are you actually using the vehicle for ? Even a light truck with these tires should have their pressure varied by the work they do. If this is primarily a commuter car, 32 lbs is fine. You start going to higher rates on LT tires on a Laredo, you loose that “velvety ride” they were designed to have. No one can give you a correct answer on this hybrid. It’s trial and error.
My suggestion is that you get a tread depth gauge and use it frequently. When you start to see a trend, say the middle treads wearing faster than the outer, then reduce the pressure. If the outer treads begin wearing faster than the middle, then increase the pressure.
Good suggestion keith.
I follow that. I follow that Thanks. I am however downsizing to 30x9.5 It will handle better…
Its lifted…I think. I crawled around underneath it to check and change all the fluids but didnt take time to look at the suspension. Its spongy feeling. I want to get it in the shop and pull the shocks and see what they feel like…
I would like to see load/pressure graphs related to even tread contact. Weigh at each corner, adjust pressure accordingly. Would the old “tyre chalking” method be useful?
I am NOT a fan of using the chalk method. 2 reasons:
- Tire wear is realtively disconnected from inflation pressure changes. There are things that have much more effect on tire wear that will competely obscure the affect inflation is having on tire wear. That means the “chalk method” is very likely to yield false results.
A good example is the wear on the rear tires on a RWD vehicle. Drive tires tend to wear in the center - which should result in a “chalk methiond” reading as too much pressure - a false reading. The front tires tend to wear on the shoulders of a RWD - and the “chalk method” redaing would be not enough pressure. The AVERAGE works out OK, but the individual readings wouldn’t.
- There are things that are done in the design of tires to affect certain properties that will result in a false reading using the “chalk method”.
The net result is that the “chalk method” is not reliable as an indicator.
Load tables? These are standardized load tables - and those tire manufacturers who publish the tables DON’T publish anything but the “standardized table”. But vehicle manufacturers have purposely moved to using MORE inflation pressure than the table calls for - which is a good thing. I had a professor who used to say: “over-design / Under-utilze!” - and that means that both ends of the equation need to do the smart thing and not push the limits of technology.
That means you can NOT use the tables directly. If you want to use the tables, you have to factor in the vehicle manufacturer’s factors that resulted in what is listed on the vehicle tire placard. Personally, I think that tire wear is a very poor way to figure out the inflation pressure, because inflation pressure goes DIRECTLY to endurance (meaning casing integrity or lack of structural failure).
Nope, I don’t think the chalk method works well enough to be considered reliable.