mythbusters tests tyre / tire pressure

[CapriRacer]

Originally Posted by UFO

But what does it actually say? To me it looks like it's not rated for more than 35psi, so that should be the stated maximum......

And yet many tire manufacturers will print a maximum pressure of 44 psi or 51 psi on the sidewall for these tires. Those pressures are permitted by the "Notes on page 1-34" - and that's my point. The maximum allowable pressure is not the same as the rated pressure.

[Bill in Houston]

Originally Posted by CapriRacer

So referring back to the page out of the Tire and Rim book (www.barrystiretech.com/2005traloadtable.jpg), if the sidewall of a P225/60R16 were to say "Max Load 1609# at 35 psi", that would be correct, but it could also say "Max Load 1609#, Max Pressure 44 psi", or "Max Load 1609#, Max Pressure 51 psi."

Not on the same tire, though. It can be an SL that maxes out at 35 (just like it says on the sidewall), or an XL that maxes at whatever is on the sidewall. I really think it's only confusing/misleading if you want it to be.

Put another way, the maximum load carrying capacity occurs at 35 psi. It is permissible to use higher inflation pressure as specified by the sidewall maximum.

This is a false statement. The tire will only have one pressure on its sidewall. That is the intended maximum. I know that table goes further, beyond 35, but that is for a tire that is built differently. It is for the XL tire, not the SL tire.

[CapriRacer]

Bill, UFO,

I think you are saying that tires built to this standard will only show a sidewall ptressure 35 psi. If you think this, then I want you to go down to either a GM dealer or a Ford dealer and pick out a midsize or full sized car (not truck!) and examine those tires. If the tires are NOT a Michelin, a Uniroyal, or a Goodrich, you will find the tires say something other than 35 psi max.

Allow me to explain why the above paragraph will prove you wrong.

1) The page I published is from the US tire standradizing organization, and while some vehicle manufacturers use that standard, the only ones I am absolutely sure will use it consistently are Ford and GM.

2) Why mid size or full size? Because both Ford and GM cars on their lots that were not designed in the US and may not be using P metric tires - but they would all be small cars.

3) Why car, not truck? Beacuse some full size trucks come with LT metric tires - and that's a different standard all together.

4) Why not the Michelin group (Michelin, Uniroyal, and Goodrich)? Because they are the only ones who will show 35 psi on their S and T rated tires. Everyone else will show 44 or 51 psi!

With those restrictions, you will find tires that were built to the US tire standards that have 44 or 51 psi listed as a max pressure on the sidewall - not 35 psi.

Ergo, the rated pressure of 35 psi for P metric tires is not always written on the sidewall.

[UFO]

CapriRacer, it would really help if you could show an example of a tire with a printed pressure rating that exceeds the actual rating of that specific tire. I read what you are writing, but I don't believe it.

[CapriRacer]

Originally Posted by UFO

CapriRacer, it would really help if you could show an example of a tire with a printed pressure rating that exceeds the actual rating of that specific tire. I read what you are writing, but I don't believe it.

Please be aware that I am using the quicklly available resources and I am NOT verifying if they are correct.

How about the tire on the 2011 Chevy Mailbu 1LT? (Source: Tire Rack) It is a P215/55R17 93S Firestone FR710. It shows a max load of 1433# with a max pressure of 44 psi.

I can show you a page from the 2005 Tire and Rim Yearbook that shows the load table that stops at 35 psi with a 1433# max load. You'll have to go to the Chevy dealer and actually look at the tire to verify that it says 44 psi max! - or you can take Tire Rack's word.

[SentraSE-R]

I guess we're quibbling over semantics, and/or fussing over the fine print. The bottom line is, an SL tire attains its maximum load capacity at 35 psi, and an XL tire does it at 41 psi. Going to the sidewall max (35, 44, or 51 psi for SLs, 41 or 50 psi for XLs) is both legal and safe, and within design parameters. Therefore, it is not overinflation.

While some people think higher inflation pressures increase a tire's risk of failure, there has been no statistical proof presented to support this theory. Product liability and company reputation considerations, combined with sound engineering considerations, all argue against the validity of the "inflation pressure relates to failure" argument, as long as we're talking about pressures under sidewall maximums.

If UFO doubts the existence of 44 psi or 51 psi sidewall labeling, a visit to any tire dealership should settle those doubts - or just stroll through a parking lot.

[Frank Lee]

As far as I'm concerned, the answer is somewhere between 35 and 50 psi for most vehicle tires and one won't go wrong running at either end of that or anywhere inbetween. Below 35 and r.r. starts to rise; above 50 and the r.r. gains are minimal while the downsides of rough ride and uneven treadwear increase.

However if one runs at 50 psi the tires should be checked for center tread wear and psi lowered accordingly.

If I was airing someone else's tires i.e. Mom, GF, grandparent, etc. I would tighten up that range to 35-40 psi as being the answer to just about all scenarios.

[Bill in Houston]

I pretty much have no idea what you are tryng to say. Show me a photo of a tire with two different psi numbers on the sidewall, and I'm back in. Until then, I'm out. 44psi and 51 psi max tires definitely exist, but I don't think that there are any tires with two different numbers molded on the sidewall.

[Thymeclock]

Originally Posted by Frank Lee

Below 35 and r.r. starts to rise; above 50 and the r.r. gains are minimal while the downsides of rough ride and uneven treadwear increase.

I'd be more concerned with handling that becomes unpredictable and unsafe due to overinflation.

Of course for those who never drive at speeds faster than 30 MPH it might not matter...

[Frank Lee]

Originally Posted by Thymeclock

I'd be more concerned with handling that becomes unpredictable and unsafe due to overinflation.

Of course for those who never drive at speeds faster than 30 MPH it might not matter...