Tom, I posted this today on yahoo:

For what it's worth, I have my tires at 44psi. If I had a rollover or
accident caused by an over inflated (above 44psi) tire, I couldn't
blame anyone but myself. I think the safety factors are up to the
designers and if they put Max 44psi Cold Tire Presure on the side of
the tire, they can be held liable if the accident was caused by tire
presure in compliance. I certantly could be held negligent and
responsible for the deaths or injuries of such an accident if I exceed
the Max presure. For that reason, 45psi is to much for me.

Same thing for Ford, they put a safety margin of 35psi instead of
44psi like the tire manufacturer. They put distance between the tire
manufacturer for liability reasons I would think. When I see a Max
tire presure on a tire of 35psi, you know Ford will recomend 32psi or
lower.

If an accident was caused by an under inflated tire, same thing could
apply. At 44psi, I have a safety margin for under inflated tires. If
an accident investigator checks for under inflated tires left with air
still in them, you would hope they would show proper presure and not
under inflated. If the one tire that cause the accident was low, well
you might not have all the personal liability because of a leak.

GaryG

 

Hi,

Sorry, I was referring to a thread (I can't seem to find it now...) that was talking about actually turning the key off the car (they were talking about both hybrids and non-hybrids) to shut everything down as oposed to letting the computer shut down the ICE and go electric. They were even talking about how many times you can use the breaks before the hydrolic assist would give out and how difficult it was to steer without the power assist! I should have commented there as this is off topic to the tires (sorry).

What you say about higher pressures goes against my memory and what I understood to be conventional wisdom. When tires are chronically underinflated, they are expected to wear more on the outsides as the shape of the tire profile changes and the car leans on the tire edges more. They also increase friction and heat and lower fuel economy.

Conversely, when a tire is over inflated, friction is reduced so economy improves. However the middle will bulge out a bit more and take on more of the weight of the car, and thus wear faster. Also, since the tread is not being distributed evenly on the ground, handling will degrade (see http://www.gmgoodwrench.com/TipsAdvi...Pressure.html).

If someone has a good reference to the contrary, I'd like to see it as I am happy to improve my fuel efficiency but not at the expense of my tires or safety.

Thanks,

Roch

 

The Ford Engineers and Designers I spoke with in Dearborn had no concerns with my tires at 48psi. The only trade off is less comfort, and less traction over snow or sand. If you have none of each, then no worries.

The weight of the car is transmitted down the sidewall of the tire, and into the corners of the tire. For this reason, tire tread wears out first from the sides.

Increased pressure will only lenghthen the life of your tires...
If you can put up with a few more bumps in the road.
Also, for light duty trucks, they always say to ADD pressure when loaded.
If it's okay to run with 44psi when loaded, it should also be equally safe (from the tire's point of view) to run 44psi all the time. What if you're loaded all the time?

Ford thinks the tires will last 75,000 - 80,000 miles.
It will be neat to see ( in 4 years ) if my tires with extra pressure exceed that.

-John

 

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GaryG,

The difference between the recommended pressure by Ford and the tire manufacturer is because Ford gives a pressure specific to the vehicle and tire, while the tire manufacturer makes the tire for many vehicles and can gives a max pressure which applies to all vehicles. I don't think it is to address handling or tire wear, just the maximum pressure that the tire is rated to stand without risk or a blowout (my impression from a quick google search).

John,

I'd love to talk to the engineers about their tire recommendations. The weight of the car is not supported by the sides of a tire. The tire is designed to hold the air under pressure. The air pressure inside the tire, which exerts the force uniformly along the walls of the tire, is what holds the car off the ground. When the tire is underinflated, the balance of forces between the tire structure and the air pressure will change and the tire will sag. When it sags, the inner tread bulges in and the outer edges make more contact and thus wear faster. Overinflating a tire will cause the oposite effect. I suppose the key in our discussion is how much pressure is required to cause a significant effect on tire wear and handling. Maybe the max pressure reflects the amount of air that will not cause that to happen. If that's the case, then the higher pressure is good (economy benefit with no drawback other than ride comfort). Your discussion wit the Ford engineer certainly supports that probablility.

I supopose the recommended tire pressure includes a fudge factor to ensure that if the car were under max load, the resulting tire pressure would not exceed the max. If you are riding at that max pressure, are you being careful not to add any weight? What if you have 4 passengers or cargo? Do you remove air, or go above the max?

Thanks,

Roch

 

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OK, I'm quickly becoming the "bad guy" here. I'm searching and reading, however much of the tire pessure stuff (re: how high should we go? tire pressure that is...) is not very scientific. One person's test on wet roads or hearsay about what NASA engineers are doing does not convince me to ignore the manufacturer's specs. A few psi here and there is OK, but some people are really taking it pretty far.

OK, enough said. I have more reading to do before I decide what I should do. Thanks for the pointers.

Cheers,

Roch

 

Wayne... why do you say that?
I'd say you are in a slight disadvantage with the ICE off... if that.
-John

 

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Wayne:
You are saying that coasting in neutral, ICE off at speed is dangerous as one runs the risk of getting their *** run over, is that correct?

MM