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Old 07-01-2019, 10:22 PM   #24
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

Originally Posted by CapriRacer View Post
That would seem to confirm that it is the 15" option that has the pressure split

Oh, if it were just that simple. The tire manufacturer is the one who decides what max pressure is to be imprinted and he is constrained to use those 3 pressures, and by the speed rating. You will find S and T rated tires with a max pressure of 44 or 51 psi, even though the speed rating test is performed at 35 psi.

No, this is the height of capitalism. Making money off of selling information.

No, the formula you shared is tantalizingly close. It just uses some faulty assumptions. The result it provides will be within shouting distance of what it would get if it hadn't made those assumptions.

Then there is the issue of what people like in the way of vehicle handling. When I was calling on Ford, I talked to the ride engineers - the ones the specify the spring, shock, and sway bar settings. Their job was to ignore their own preferences and delivery a product that met the goals as determined by the management team. But one of the items NOT on the agenda was tire pressure. That had been predetermined based on a procedure the company had loads of experience with.

However, that result of tire pressure, spring rate, sway bar size, and shock damping didn't always appeal to every driver and it is common for folks to tune their car by using a tire pressure different than what is on the vehicle tire placard. The danger is not using enough tire pressure - which increases the risk of a structural tire failure - or using too much that wet traction is significantly reduced.

So, as far as my friend's 2005 Subaru Legacy GT is concerned:

I checked the size listed on the GY Eagles currently on the vehicle: 'P215/45R17', load & speed index: 87W.

87 load index = 1,201 lbs.

Now here's the kicker:
Her door placard lists original tire size: '215/45ZR17'.

So her current tires are not of the original Z speed rating, but are W. Probably saved whoever had them put on at least $50 per tire! This is a 15 year old car, so the exact size listed on that door placard is probably no longer available.

My smaller Hyundai's OEM T-rated(lower speed rating than W or Z!) tires have a higher load index - 91(1,356lbs) - on a smaller car! As well as my wife's: H-Rated 91's. That likely explains why the formula worked for both our rides, and why it didn't, initially, with my friend's Legacy GT. It's got the wrong tires on it! LOL

I've already run the formula under alll the scenarios, several posts above, so I'm not going to repeat here, but I do know that I got more reasonable results for that particular Subaru with the correct load & speed rated tires plugged into it.

Last edited by RidingOnRailz; 07-02-2019 at 10:16 AM.
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