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Old 06-30-2019, 11:21 AM   #22
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

See my 'Railz' responses, within:

Originally Posted by CapriRacer View Post
No, I'll be accusing Tire Guides of making a mistake in their book. This wouldn't be the first time I found one.

I wonder what the placards actually say for the other versions. Put another way, it is unusual to specify more inflation pressure for the rear tires on a FWD, and if all the Contours had that pressure split, why did Tire Guides only indicate it for the one combination? I wonder if it was that way for the 1995 models and the 15" - which only came on the 1995 SE model - had that pressure split and Tire Guides just didn't pick up on the change made in later model years.

Railz: My 1996 GL had 205/60R15 tires on it. At this point I don't recall the speed rating, but I replaced them with H-rated, same size.

The tire standardizing organizations publish load tables - max load vs inflation pressures - in the form of a book published every year. While a tire manufacturer is not under any obligation to follow those, not only would it be foolish not to do so, there have only been a few cases where the tire manufacturer did something other than what was published - and eventually, those exceptions disappeared.

The US government accepts those standards and requires the tire manufacturers to imprint the max load and the max inflation pressure on the sidewall of each tire designated for street usage.

And here's where the confusion comes in: Officially, all Standard Load passenger type tires will have their load max out at 35 psi (or 2.5 bar (36.3 psi) if the standard is written in metric units), the tire standards allow 44 or 51 psi (3.0 bar and 3.5 bar) to be used as well - and the tire manufacturer deicides what to place on the sidewall. The sort of exception to this is that for the speed rating test, while S and T rated tires are tested at 35 psi, H rated tires are tested at 44 psi and V and higher rated tires are tested at 51 psi. Needless to say, tires with those speed rating HAVE to have max pressure of at least that value.

Note: The speed rating test is the only standard test where the inflation pressures are as indicated in the above paragraph. Standard load tests use 35 psi or 2.5 bar, whichever is appropriate.

Railz: So the max cold stamped on a tire(35, 44, 51psi) is based on that speed rating?

Unfortunately, those tables are copyrighted and can not be reproduced without permission - which I don't have. You will occasionally find portions of those load tables published by tire manufacturers on the web. But since there is a yearbook published every year with new sizes (and old sizes removed), you will not always find a particular size - although the information on a given size doesn't change from year to year.

Railz: Communists!

Besides, the current TRA yearbook has 60 pages for the passenger tire section alone. For me to publish those would be a lot of work and I risk a copyright infringement lawsuit. Not to mention there are 3 major tire yearbooks in common usage with about the same number of pages - also covered by copyright law.

However, I did publish a few pages from the TRA yearbook for educational purposes (allowed under copyright law), and pages of old yearbooks for obsolete tire sizes - like size G78-15 or 7.35-15. Those old pages don't represent much of a legal risk as TRA would have to show monetary damage to their business and information about old tire sizes just doesn't sell.

Now allow me to return to an earlier conversation we were having:

At one point in time, there were only 3 speed ratings and Z was the highest - above V - and the speed rating was to be imbedded in the size designation. But when W and Y speed ratings were added, the Z speed rating couldn't be changed, so technically Z speed ratings include W and Y, but are further restrictions - plus it is still permissible for tire manufacturers to place the speed rating in the size if they want to, but with the advent of service descriptions, there is no longer the need - EXCEPT, it seems that some tire manufacturers imbed the Z in the size for W and Y speed rated tires. I don't understand why.
Railz: I will have to look at the size section on her sidewall again for that load index and speed letter, IE: '89T', 91V' or which ever hers might be. BTW on my Contour, the replacement tires did have the H embedded inside the size, and they were in fact H-rated tires.

So Capri: Would you consider it just happy coincidence that the formula I shared here worked for mine and my wife's cars and tires, yielding as it did a perfect split just above and below the equal recommended tire pressure on our door placards? Because we both really like how our cars drive after setting them to the values I derived.
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