Scoured the net; searched this forum.. the answer remains elusive:
What is the correct "factory" tire pressure for a 2000 Jeep Cherokee Classic with 225/70/16 tire size??
And please don't tell me it's printed on the tire.
Anyone?
The "factory" tyre pressure is usually either on the tyre sidewall or on the data plate in the driver's door jamb. However, it's usually
not "correct."
The "chalk test" is the most accurate way to determine your ideal tyre pressure, with your typical vehicle use & loading. The tyre pressure on the sidewall is the
maximum for that tyre with
maximum loading, the "factory" tyre pressure is for
factory tyres & loading.
By the time you've got a couple hundred K on the clock, both data points are invalid.
The "Chalk Test"
What you need:
- "Railroad crayon" or other large piece of chalk in a bright colour (sidewalk chalk also works.)
- Tyre pressure gage of known accuracy (I find the "dial type" gages hold their accuracy well, provided they're not dropped. I keep one in each vehicle, with correct determined pressure on a label 'round the edge.)
- Tyre pump, to adjust pressure.
- Clear road (about a block will do - you don't need much.)
- Clean rag & water.
- About an hour (probably overkill, but you'll be playing around a lot.
What you do:
- Prepare your vehicle with a half-tank to a full tank of fuel (gasoline runs about 6#/gallon - therefore, testing with an empty tank is generally inaccurate.)
- Inflate
all four tyres to nominal maximum pressure.
- Mark
all four tyres with a broad (~1" wide) chalk mark across the
full tread width (I find it helpful to spill over onto the outer sidewall, so you know where the mark is.)
- Drive about a block - speed irrelevant.
- Stop, roll until all four marks are exposed.
How to "read" the marks:
- If the mark is worn off at the ends more than the center,
increase pressure.
- If the mark is worn off at the center more than the ends,
decrease pressure.
- If the mark is worn off evenly,
maintain pressure and retest. If the mark still wears off evenly, you've found it for that corner.
It is common for the "ideal" pressure to vary by as much as +/-3-5psig at each corner, but it will usually be close (tyre pressure and weight are the primary factors.) You can either average the pressure and use that for your "nominal" reading, or you can actually note all four corners for individual pressure, to maximise tyre life. However, given that the "ideal pressure" usually falls within a range of 3-4psig, averaging it out is simpler, and the difference in tyre life is nominal at most (unless there is an exaggerate difference between load on each tyre, as with on work trucks and suchlike. I once drove a work truck that wanted a 15psig difference between front & rear axles! Then again, it was a 10,000#GVWR vehicle, loaded to about 11,500#...)
Any "book value" you find is likely to be invalid - the "book values" are fine for when the book is written, but you've changed all that. If you find that you can't get enough pressure into the tyres before you hit the max on the sidewall, change to a tyre with a higher load rating.
The reason no-one can give you a proper useful answer on this question is because it's one you have to find for yourself. Changing loading will usually change ideal tyre pressure. Changing axles will usually change it as well. Cut off the back? Change. Add ballast? Change. Different tyres? Change (even if you use the same brand/type in a different size!)
At least the test to find the proper tyre pressure is easy - takes about an hour (or less,) a big-ass piece of chalk, and maybe a gallon of fuel (more than it used to cost, but still cheap enough.)
And, you can
only determine this practically - you can't accurately calculate it, and you can only "guesstimate" it otherwise at best.
EDIT - Other things to keep in mind:
- The pressure gage on most filling station air compressors is wildly inaccurate at best! I check pressure with my gage, note the reading on the shop gage, and use that to estimate where to stop filling. Go a bit over, put my gage back on, and bleed down (it has a bleeder button just for that purpose - allows a controlled deflation with constant monitoring.)
- Do this test "cold" - know that the internal pressure of the tyre tends to increase with long drives (which is why I don't inflate tyres extra when I know I'm going on a long trip - they'll heat up and do that for me.) If you look at the sidewall, it's usually something like "Max. Press.
xpsi at
y lbs. cold" - I've seen "hot" pressures (long drives) increase by as much as 25% over "cold" pressures, depending on weather, road conditions, road speed, &c., &c. What's that old equation? PV = rT?
Here it is - PV=nRT (the "Ideal Gas Law.")
- P = pressure of gas (Pa)
- V = volume of containment (m^3)
- n = "Amount of substance" of gas (chemically relevant, but considered a constant for this purpose) (moles)
- R = "ideal gas constant" (unimportant here - it's a constant.)
- T = temperature of gas (*K, *C+273)
Tyres head up due to friction with the road surface - varies by surface consistency, surface material (concrete/asphalt/gravel/dirt/...) and absorbed solar radiant heat and ambient temperature. Recall your algebra, and then imagine what happens when a single value in that equation changes without altering anything else (V, n, and R are effectively unchanged - therefore; if T increases, P will increase; if P increases, T will increase. Probably been a long time since your last physics class, but that doesn't mean that physics is held in abeyance. Gimme a break - I'm an engineer by nature!)
Ref: "Ideal Gas Law" for further information.
