Yet another tire pressure item/caution

I keep mine at the recommended pressure, too. I've never seen anything but anecdotal evidence that higher pressure increases gas mileage. And, if you note by my signature, I do pretty well MPG-wise.

Thanks,
Shawn

 

Ok, since this is a subject near and dear to my hear I just have to comment. I too believe we're just a bit misguided is our excessive use of pressure to boost gas mileage. I have to admit that even though I've taken very carefully controlled data that tells me 42/40 is only worth, at best, about a 3% gas mileage improvement, I have generally used the higher pressures. As an engineer I take all this enthusiasm for increased pressure with a grain of salt and "say show me the engineering data". I've found lots of data online that shows the rate of increased gas mileage with increased pressure decreases at higher pressures. In other words, underinflation will hurt your gas mileage much more than over inflation will help it. A quote from one of the Goodyear sites states that they've found that that average gas mileage reduction of an 8 psig under inflation on a large group of modern vehicles they've tested indicates there is a corresponding 3% average decrease in gas mileage.

And while rereading something I had copied awhile ago about Goodyear's E-metric, a low rolling resistance tire in development, I came across something that supports some of what I believe to be true. This tire is being designed to work on the coming fuel cell vehicles as well as possibly on some hybrids in the future. It's designed to run on a special 22 inch rim at 51 psig max pressure. It's a very interesting looking tire. It sort of looks like a cross between the Michelin Hydroedge and something you might find on a very high performance motorcycle. Here's an excert from the article:

But higher pressure in ordinary tires would result in rock-hard ride with potentially poor handling. "You can't just jack up the pressure, you get too small a footprint, says Bill Egan, chief engineer of advanced tire technology at Goodyear. "We developed a new tire profile. A narrower rim width and rounder sidewalls make the tire more shock absorbent"

The article goes further to say that the tire is rated for 51 psig but most electric and hybrid uses will be under that number. Now don't you think Bill is somewhat of an authority on tires?

There are a bunch of technical articles out there about reducing rolling resistance and how much it savings it could generate. The auto industry along with the tire industry is very very eager to do this of course. If it was as easy as just jacking up the pressure to the max stated on the tire, without any undesireable consequences, every knowledgable tire engineer would be advocating that and they're not doing that.

By the way, I've really experienced that rock hard ride they're talking about with my new Hydroedge tires at 42/40. It's even generated a couple new sqeaks and creaks in the dash. So it's back to a more reasonable pressure for me I think...maybe 37/35 but no more than that. I did try the 195h60X15 Hydroedge tires for about a month and elected (maybe in error) to go back to the std size. I think the large size hurt the mpg's somewhat but really did improve the freeway handling a lot now that I've tried both of them. I have not tried the lower pressure with the new tires so maybe that will work better.

Fred

 

I think this may have been mentioned before, but it's worth repeating. Toyota delivers the Prius with 45psi in the tires. Sometimes the dealer neglects to reduce this pressure before delivering the vehicles to the owners. So for new Prius owners, make sure you check your tire pressure before driving the car too far.

 

I have followed advice in this forum and use 38/36. I wonder to what extent the high tire pressure is responsible for the myriad of creaks, squeaks and rattles produced by this car over uneven pavement. Ny 2004 Prius rattled at 6000 miles twice as much as my 2002 VW Golf at 23,000 miles.

I like the Prius a lot but I don't like all the rattles.

 

i used 38/36 and according to my brother, you can easily exceed the tire pressure max by 50% in nearly all conditions.

the pressure is set to make the tire conform to shape (overinflated tire bulge and therefore not all the tread makes full contact with the road) ride comfort and the built in safety margin for "extreme" road hazards.

as it is, i average 53.2 mpg and although higher would be nice, im not willing to compromise any safety margins for what i perceive to be minimal gain.

p.s. the reason i have it at 38/36 is because that is the way it came straight from the dealership.

 

I would not consider that way to high. But if you try 42/40 or go to a stiffer tire like the Hydroedge at 42/40, watch out ....it will creak more. Yeah, some of our cars just appear to do a lot of creaking and some say they're dead quiet. Count mine as the creakiest car I've ever driven in!!!!

Fred

 

I have posted on this subject and on Tom & Ray before so I will not repeat myself om old territory.

Cars generally handle better with higher pressure because the sidewalls flex less. The tires will also tend to run cooler because of this reduced flex.

I think that Tom & Ray are interesting, but have made enough technical errors to actually cause some controversy instead of solving it. Well, nobody is perfect, and certainly not I.

The desired tire pressure for a car depends on many factors, many compromises. The factory tends to set them for a comfortable ride, and yes, I have called and spoken to most of the major tire manufacturers this year on this very subject.

What makes it most difficult to state an absolute number for pressure is that it varies by tire model (hence so many complaints about the Hydroedge), load (car weight + weight of contents), speed, temperature, surface and most importantly (to me) driver style/preference.

What I find most interesting is that underinflated tires have caused many tire failures and subssequently many accidents yet I don't hear Tom & Ray standing up on their soapbox making daily pleads with their audience to up their tire pressure to at least the factory compromise. I think that I would make a lot of noise about underinflation if I had their soapbox (at least I like to think that I would do something about it).

 

I agree with you about under-inflation. And that's probably why it's so easy to find so much information on it. It's much harder to find much about the benefits of running at the max pressure ratiing of the tire, or way beyond what the tire manufacturer recommends. But there is some info out there like what I offered from a chief engineer at Goodyear. And there is data about the diminishing returns regarding over-inflation.

My take on this is that nothing is infinitely stiff or resistant to stress...and that includes the contact patch on any modern day radial tire. So there's a pressure that will maximize the contact patch of a specific tire for a given load. Deviate from that ideal pressure and you will affect the siae of the contact patch as the engineer from Goodyear stated. Now that deviation may be slight within a specific range I'll grant you that. At the extremes, 42-44, I've been able to detect more wander and less stability, at least with my car. And at 42 psig, with the Hydroedge, I don't know anyone that would not say that it feels extremely harsh, even with the soft suspension of the Prius. It's like riding on rocks, period. And it caused at least one new major rattle on the dash of my car that went away at 37/35.

I'd just like to say that if inflatiing our tires to the max pressure of 44 psig was such an obviously good thing, then everyone of the tire/auto experts out there would be pushing that hard. If harsher ride were the only issue they'd probably add a disclaimer stating that. So if you know any of any automotive experts (true experts) that advocate this or of any data supporting this please would you share it with us all because I could not even find a single one.

Fred

 

Overinflation is an act of Commission, Underinflation is most often an act of Ommission.


Part of the reason underinflation is talked about so often and little is written about overinflation is probably because people rarely check their tires. So just the normal losses that occur over time lead to an underinflated state. It always amazes me at how often I find flattening tires, less than 20 PSI, while wandering in my office parking lot. (I have a back door, and often "escape" to dictate undisturbed during the day). Once, even after informing one of my office staff of her flattening tire, it took the better part of a week for her to do anything about it, and it was only after she subsequently caught me trying to get a little air back into it with a bicycle pump!!!

 

lol...jc, your dedication to educating the unimformed is admirable.

 

<div class='quotetop'>QUOTE(jchu\";p=\"38996)</div>

I've read through more than a few technical papers on rolling resistance. The automobile companies of the world, and therefor the tire companies, have been doing lots of research the last few years in an effort to reduce rolling resistance and boost fleet average mpg numbers. The use of advanced materials like Silica and long chain rubbers have really reduced the rolling resistance in tires from Michelin and Bridgestone and probably others. This is not an easy problem because of trade offs involved. And that's usually handling, tire life, and a very harsh ride.

Your conclusion about people not writing about over-inflation because people rarely check their tires just doesn't make a bit of sense to me. The engineers, auto experts, etc. don't advocate extreme over-inflation (at or close to the advertised tire max) because the data shows there to be marginal returns and a very likely reduction in contact patch resulting in diminished handling/stability. Granted, some drivers might not notice this unless they're in a situation that demands the maximum performance out of the tires.

The lack of data to support the claims of up to 10% (or more) mpg improvement doesn't seem to deter the "perpetual motion" advocates one bit. Data that goes contrary to popular belief here is totally ignored because of the desire to want to believe the amazing overlooked (by the experts) benefit of extreme over-inflation of tires. Maybe it's the placebo effect, lol.

Heck, I didn't even see any blip in my mpg numbers when I ran 35/33 in the 195H60X15 Hydroedge tires I had on my car for 1300 miles! I also ran them as high as 44/42. And I saw zero change when I went to Mobil 1 synthetic oil, another area where some people claim 1 1/2 mpg or more! But still my limited test data implied as much as a 3% improvement at 42/40 over 35/33 with the stock tires. I just don't have total confidence even in my test data because of all the variables involved and the limited number of data points. But my data is not unreasonable when compared to what I've read from the experts on the subject.

Fred

 

Fred,

I think you missed my point. The point that I was trying to make in my opening sentence is that to have an overfilled tire a person has to willfully put extra air into it. If one just ignores ones tires, eventually they will become underinflated due to normal losses around the stem, bead, and through the rubber itself. in other words Commission vs. Ommission.

Does this help you understand why I think underinflation is written about more. Most people don't check thier tires regularly which ultimately leads to low pressure and thus a much higher percentage of tires on the road today are underinflated rather than overinflated.

No comment was made in my posting about the merits or dangers of overinflation, only why I think that it is a less covered topic.

Jon

 

jc i understand your point and the commission/ommission is as astute as it is accurate an observation regarding how many people view tasks.

i find myself equally divided between both (as i feel most people do) and i feel somewhat guilty for being so.

so in effect, the jiffy lube people could be just proactive in making sure that the recommended operating range is maintained a bit longer. after all, in the true light of day, a few pounds aint gonna make a difference anyway. especially considering on a summer day, 20 min on the freeway can increase tire pressure by 20 psi.

 

Dave you would be amazed at how often I've heard of people who don't change their oil either!! And radiator fluid not a chance. Of course the radiator fluid issue is different for the Prius.

 

<div class='quotetop'>QUOTE(DaveinOlyWA\";p=\"39178)</div>

Actually the rise in pressure when you are driving is somewhat irrelevant to the cold inflation pressure. All tires will increase in pressure as you drive the car (and it can get much higher than I think most people imagine, far higher than 44PSI). The higher the initial cold pressure, the less the tire will flex, and the less heat the tire builds up and the less the pressure it will increase. This is all because there is a point of equilibrium that is achieved, regardless of what you do. The tire will keep flexing and keep getting hotter and hotter until this point is reached.

Another upside of increased tire pressure is lower temperature rise/less tire flex and subsequently longer tread life or higher grip (depending on how you are driving).

I've gone on about this issue here and on Yahoo groups but I don't think that I made it clear what i actually do. I raise the pressure to a comfortable high level (in this case its 44PSI in my Prius) for me. I then check the tire wear with a small guage that measures the wear in each groove. If I detect that the wear is uneven (not happened in the past 10+ years of using mostly Michelin radials) then I would drop the pressure down.

All the cars I've done this with for the past 20 years have experienced a small increase in fuel economy and I seem to get longer wear than any of my friends using the same tires.

As for ride, I find that the Prius has a somewhat jiggly ride on certain surfaces. This is one three different cars, mine included and raising the pressure has increased the effect slightly, at the benefit of eliminating the wandering I was getting at highway speeds. Certainly a good tradeoff for me. As for MPG improvement, its too early to tell as the car is so new but it seems to be a solid 2MPG better. Since the Prius is the culmination of many small improvements, I am happy if I can find one or two more to squeek a little bit more out of her.

 

the ride is a bit stiff, but some seem to think that tires play a very small role in the effect. to improve aerodynamics, the car is set to ride somewhat lower with stiffer springs as a bouncy car will degrade mileage also.

i did test this theory a short period of time (actually only about 10 miles on roughest freeway i could find in the area and living in state capital, our freeways are in pretty good shape) i set inflation of all tires to 30 psi to see if there was a difference and i didnt notice a thing. but that might just be me... i should try the test again.

you see, i did this during my first tank of gas and i have to say that i probably wasnt used to the car at first. realize that i had been driving a 1991 ford F-150 so the Prius felt like a cadillac ride. i frequently found myself doing 75 mph without realizing it because the ride was so smooth.

today because of heavy rains, i was thinking about reducing the pressure in the tires again to test traction and skid control since we have a few places on I-5 near the HWY 101 interchange where water pools and creates a severe hydroplaning effect. but i quickly realized that there is no real sane way of testing the difference, so i bagged the thought...

 

<div class='quotetop'>QUOTE(DaveinOlyWA\";p=\"39232)</div>

I am so glad that you came to your senses in time Please see the Michelin photographs that I posted on another of these tite pressure threads to se what happens with underinflation and water (not pretty). Their photograohs clearly show that the higher the pressure the better the contact patch in the rain.