Quote:
Originally Posted by RidingOnRailz
Well, according to tirepressure . com’s old vs new tire calculator, when I put in “P225/55R17” and Standard Load for existing tire, and Metric 225/50R17, standard load for replacement tire, the converter calculated a cold pressure for the replacement tires 1psi higher than for the P-metric. 33 vs 32.
Their website, their calculator, not mine. Same happened for my wife’s tires. 31psi for metric vs 30psi for OEM P-metric. Indeed, a “slight difference”, as you yourself stated. But a difference, nonetheless.
Even in your examples on your own website, going from a P-metric tire to a Metric resulted in a higher tire pressure to carry the same load as before.
Grass is green and sky is blue. I report what I see and what I experience
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First, you changed sizes: 55 vs 50. That's not the same as going from P metric to metric in the same size.
Second, there is a problem with the calculator. It assumes that each way of doing the calculation is correct - and that can not be. The physics of the situation is that, physically a P225/55R17 is the same as a 225/55R17. The difference is in what label is applied to the tire - and the labels are different. From there would be testing to assure that the tire passes the test.
Put another way, if I gave you a tire without anything written on the sidewall, and TOLD you it was a P metric, you should test at the P metric conditions.
If I gave you the same tire and told you it was a metric tire, you should test it at metric conditions. In both cases, you'll get the same result - the tire fails in the same manner and under the same conditions.
Ya' see, each standard is trying to describe what is going on with the tire, but the way each standard is done is different, so you get slightly different answers because each way is imperfect in its description (the formula!)
Yup, the math comes up with a different pressure, but that's because the math is imprecise.
Want a further complication? There is a third standard - JATMA (Japanese Automobile Tire Manufacturers Association) - and it's standard (the formula) is different still. This standard has sizes that look exactly like the metric standard - 225/55R17 (no "P") - and some people have a different word to describe what is published by tirepressure.com - Euro-metric - where they call what is published by JATMA - hard metric. BTW, what tirepressure.com is using is from ETRTO (European Tyre and Rim Technical Organization).
Ya' see, this is all very imprecise. As I said before, each standard is trying to describe what is going on and each standard isn't very precise when they do that. That's because each of these standards was created BEFORE there were digital computers capable of doing more detailed analyses.
Just as an example of this imprecision: tirepressure.com lists their metric load tables using English units. The standard is actually done in metric units - BUT - they did the load OK, but the pressure is off. What they list as 36 psi is really 250 kPa (36.3 psi). Again, imprecision.
But in the big scheme of things, it doesn't matter. It's close enough.
And remember where this discussion started: using 51 psi as the rating pressure. And clearly none of the standards do that.