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Old 06-28-2019, 10:04 AM   #18
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Cool Re: For Anyone Who Owns a Digital Tire Gauge

Originally Posted by CapriRacer View Post
Remember when I said that the max pressure is NOT related to the max load? I suspect the answer you are getting for the Subaru is because the tire's max pressure is 51 psi, where the actual rating pressure is 35 psi.
Wait - HOLLLLLLLLD everything!

I just noticed something odd: The USTA load stamped on the Subaru tires(1201lbs) is LOWER than it is on my SMALLER Hyundai(1356lbs), and even smallllller Corolla, also 1356lbs. I'm surprised - and actually embarrassed - that I didn't pick up on that weeks ago!

This changes everything! Apparently her tires are NOT of the OEM specified on her door B-pillar: "215/45ZR17". I neglected to photograph the size stamped on the tires, so while I do recall the size, I don't remember exactly what speed/load ratings the tire place sold someone who owned the car before my friend! Will it be a risk for her to keep these lower load, NON-oem tires on her car, since it is mostly just her driving across town to work, and sometimes with her kid?

So I found a Continental Extreme Contact in that exact size as indicated on the Subaru door sticker, BUT: That "Z" is NOT the speed rating! The speed rating, acc. to Continental's own page for that tire, is W! With a load index of 91(1,356lbs).

So I don't know WHY there is a Z in the middle of that tire size if it does not refer to the speed rating. As Trump would say: "Misleading - VERRY misleading!"

Anywho, I ran the formula with the new 1,356lb USTA figure, and get something a *little* less truckish for my friend's poor Legacy:

F: 43psi, R: 44.

Also Capri, when I left in the original USTA load - the one actually on her car - and used 35psi max cold in the formula, I got:

F: 33psi, R: 34 (basically the opposite of her door placard: F: 35, R: 33).

When I used the correct USTA load(1,356) annnd 35 max cold(as you mentioned), I got:

F: 29psi R: 30

BOTH axles calculated under the door sticker cold pressures! Although those are the exact cold "normal load" pressures I saw on my boss's 2015 Forester. Cosmic.....

So the second to last set of pressures seem the most sensible, to me anyway, for this 2005 Legacy sedan. LOL! Notice though, how in each scenario, the formula flips the pressure difference between front and rear. Interesting.

Long n short: The tire pressure formula I shared happens to work perfectly with the tires on the cars in my household, adding pressure to the heavier axle and subtracting it from the lighter one.

Now for the greatest mystery of the tire universe:

How, in 1994, Ford came up with the recommendations for my 1996 Contour - remember, Mondeo? World car? Driver extraordinaire?

Front: 31psi, Rear: 34!

Wait, wasn't Mondeo/Contour a FRONT-drive econobox - or econobubble?

Why would Ford specify higher tire pressures on the LIGHTER axle?

Anywho, from 20 to 17 years ago, when I drove the Contour, I tried Ford's pressures for one week - and never looked back. One of my favorite drivers! If it had a moonroof, I might still be driving it today!

Last edited by RidingOnRailz; 06-28-2019 at 11:05 AM.
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