Pressure - more is better

[mva]

Lots of different opinions here. Obviously, every vehicle/tire/driver combination will produce different subjective and objective results.

I increased the tire pressure on my Honda Accord from 32 up to 44 psi all around. There is a bit more road noise, the ride is slightly rougher but still fine for me. The handling seems a bit crisper. I am curious if I will be able to detect a change in mpg. It's not that I can't afford the gas - I find it fun to experiment in improving mpg.

I will admit that Honda's recommended tire pressure probably provides the best compromise between ride, handling and fuel economy. But contrary to other opinions posted here, I don't expect any significant problems with traction, tire wear, suspension wear or hydroplaning.

1 mpg or even 0.5 mpg may seem minor but it is pretty significant if spread over a large number of vehicles. Gas here in Canada currently costs 1.25 $/litre (4.80 US$/US gallon). If a set of tires lasts 50,000 miles, going from 31 to 32 mpg will save 50 gallons of gas or $240 over the life of the tires. This is 3% improvement in mpg on a light weight econo car; the savings should be proportionally greater on a larger heavier vehicle.

1% fuel savings on all US vehicles would save up to 1,400,000,000 gallons of gas per year.

Yes, the US population burns 140 billion gallons of gasoline per year.

The point of my original post is that higher pressure combined with low rolling resistance tires can significantly improve mpg on any vehicle. And that low pressure (less than 35 psi in Tirerack's example) can significantly increase hydroplaning.

There is limited data on low rolling resistance tires but Michelin Energy series and Goodyear Viva 2's are both advertized as low rolling resistance. I havn't tried either but will certainly consider them next time I buy tires.

California is currently pressing tire manufacturers to include ratings for rolling resistance on all tires. They are trying to implement an efficiency rating system without compromising tire wear or safety. Here is an extensive report on this topic from California's fuel efficient tire program:

http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf

 

[CapriRacer]

MVA,

Before I start I want to correct the labeling on the studies you cited.

The study cited in your first post is the CA study (and I'll call it that throughout this post), and the study you cited above was commissioned by the Feds (and I'll call it the Fed Study.)

Nevertheless, in both reports you cited, there is a fallacy.

Tire wear, traction (especially wet traction), and RR are all competing for the same space. Both reports tried to examine the interaction to determine what the best course of action should be. Unfortunately, they made 2 fatal errors:

1) While they measured RR in a consistent way (They had to. There is no universal agreement as to what test method should be used, so it is not routinely done.), they did not test traction or treadwear - they used the UTQG ratings.

While the UTQG rating system specifies a test and the test is fairly repeatable, the marking requirement isn't. Marketing plays a large role in deciding what gets put on the sidewall.

Both groups looked at the data and saw a lot of scatter and concluded - incorrectly - that the scatter was due to difference in manufacturer.

The generally held opinion is that you can not change this 3 way relationship without a technology change. Silica was one of those, and it is still being worked into the system.

BTW, if you've heard about or experienced wet traction issues with some tires - silica! There are certain pavements that don't react favorably to silica.

This is part of the problem with the UTQG ratings. One of BITOG posters (GC4Lunch) has correctly pointed out that the UTQG traction rating is strictly a locked wheel wet traction test on 2 surfaces - asphalt and concrete. In these studies, the effect of traction on the wide variety of road surfaces used throughout the US isn't considered. This is a major shortcoming, as many vehicle manufacurers are started to encounter this problem as they strive to improve fuel economy by specifing lower and lower RR values.


2) Both groups did not take the next step - which was to examine what a loss in treadwear would cost in terms of energy to replace tires sooner.

Oh, they estimately some costs, but did you notice the values they got were pretty small. They did the best they could to make them significant - but they couldn't.

In fact, if you were to replace a tire with a 520 treadwear with one with a 260 (with better RR), the energy difference to make and transport the extra tire is much more than the difference in fuel economy savings.

I find it interesting that in the CA study, they spent over half of the study looking at the Public Relations campaign - costs and effectiveness.

Overall, I am dissappointed in both thse studies and have expressed this to one of the reps at the RMA - who was involved with the CA study, but not the Feds study. As he commented to me, he thinks both studies had more to do with a political agenda than actually improving things. However, he was hopeful that one defined RR test would eventually come out of this and that would allow the marketplace to decide what this was worth.

 

[Drew2000]

Originally Posted By: mva

...
1% fuel savings on all US vehicles would save up to 1,400,000,000 gallons of gas per year.

Yes, the US population burns 140 billion gallons of gasoline per year.


Consumption is in proportion to population....

Oil Consumption> Thousand Barrels Daily> Per Capita:

US: .694 per 10000 people (#5)
Canada: .672 per 10000 people (#6)

Source: BP Statistical Review of World Energy, 2005

 

[Colt]

Originally Posted By: CapriRacer
Originally Posted By: Colt
.....

The recommended was the door sticker pressure.
I'm wondering if the nitrogen's keeping the pressure when hot down compared to just air,a lb or two when hot,actually means I'm like 3-4 lbs under door number.
As a tire with regular air heats up and increases pressure more than a nitrogen fill.


This is a myth that is being spread by the makers of nitrogen generators. Nitrogen behaves just like air according the Ideal Gas Law: PV=nrT.

Even if it behaved differently, air is 78% nitrogen, so it's unreasonable to think it is going to behave significantly different that air.


interesting.
I thought race cars used nitrogen in the past because of the air pressure not rising like regular air..
Oh well,I'll continue to use nitrogen.I have a friend in a business that also sells nitrogen.I get it for free.I also have an 11 gal tank full to top off when necessary.

 

[Tornado Red]

Originally Posted By: Colt
Originally Posted By: CapriRacer
Originally Posted By: Colt
.I'm wondering if the nitrogen's keeping the pressure when hot down compared to just air,a lb or two when hot,actually means I'm like 3-4 lbs under door number.
As a tire with regular air heats up and increases pressure more than a nitrogen fill.


This is a myth that is being spread by the makers of nitrogen generators. Nitrogen behaves just like air according the Ideal Gas Law: PV=nrT.

Even if it behaved differently, air is 78% nitrogen, so it's unreasonable to think it is going to behave significantly different that air.


interesting.
I thought race cars used nitrogen in the past because of the air pressure not rising like regular air..


An increase of 3 or 4 pounds of pressure may be noticeable on the highway, if one had an objective method of measurement. Racing drivers have such a method, it's call lap times.

One reason they start with heated tires is to reduce the amount that tires heat up on the track... though I think it mainly affects tread grip.

 

[Benzadmiral]

My FWD Buick specs 30 psi all around. I bumped it up to 32 and was still comfortable driving these nasty cart tracks around here, but 34 was getting hard to deal with.

Then I had the rear shocks replaced, and the ride back there is firmer. So I'm trying 32 rear (to compensate for the firmer ride) and 34 front to balance it. So far so good.

 

[wantin150]

Capri,

What road surfaces don't react well to the silica compound in tire rubber? And why?

 

[CapriRacer]

Originally Posted By: wantin150
Capri,

What road surfaces don't react well to the silica compound in tire rubber? And why?


I don't know the answer to that yet. It's only been in the last 2 years that I have been able to determine that tires with low RR values (and therefore high silica content) tend to have wet traction problems - AND they seem to be concentrated in tires supplied to Japanese vehicles. I suspect that it has to do with the fact that all traction testing for these vehicles takes place in Japan.

What I have been able to determine is that wet traction complaints seem to be isolated and not universal. In other words, I hear folks complain about wet traction problems, and other folks say the same tires on the same vehicle are OK.

In order to determine the answer to the question, I need a better feel for what part of the country these reports are coming from. So if anyone can help me identify both the tire they've had wet traction problems with and the part of the country (Let's say state), then I might be able to isolate this.

All help would be appreciated.

 

[eljefino]

Noticed some almost polished concrete surfaces in Oneonta, NY and its surroundings.

Never pushed it hard but could feel traction was low. Rain pooled and puddled instead of running off or hiding in the (nonexistant) cracks in the surface.

It would be hard to do a fair comparison in a lab setting, as the same amount of precip would disappear from asphalt but stick around on the concrete.

 

[oilnoob425]

I increased the tire pressure on my Honda Accord from 32 up to 44 psi all around.quote]

Do You relise You significantly increasing risk of blowing up your tires by overinflation? Slight gain in MPG is not worth compromising your own and others safety.

 

[shoz123]

I always inflate the front tires to the max on the side wall. Then the rear 8 lbs under the max. When rotated front to back every 5k or so this will give me excellent and even tread wear.

My newest tires are Z rated Kumhos and carry a 51 lb max rating. Don't know where I'll end up at with these though. I had some Michelin Arctic Alpin Pilots once with a 51 lbs rating. If you ran them in the 35 lbs range it felt like riding on inner tubes.

FWIW when driving at high speeds on Z rated tires for extended lengths of time it is recommended to increase the inflation pressure beyond the sidewall rating.

IMHO the door placard is only meant for the tires that originally came on the car and are solely a ride comfort setting. Unless you have a Ford Exploder then it is a tire blowout setting.

 

[Colt45ws]

Originally Posted By: oilnoob425

I increased the tire pressure on my Honda Accord from 32 up to 44 psi all around.quote]

Do You relise You significantly increasing risk of blowing up your tires by overinflation? Slight gain in MPG is not worth compromising your own and others safety.

Only if that is above the sidewall max cold would I worry about it..
I had a car come in once that had over 90psi. And it was a 44psi rated passenger tire. No idea how long it had been that way.

 

[mstrjon32]

The most important point here (and I don't think anyone has made it) is that you are significantly reducing your grip and stopping ability by severely over inflating your tires. Higher pressure means more MPG, but it only means more MPG because there is less tire surface in contact with the ground and thus less rolling resistance. But, less tire surface in contact with the ground also means you don't have nearly as much grip when it comes time to make that panic stop.

I'd like to see someone do a test of stopping distance at the manufacturer recommended PSI and while over inflated. That should be enough of an argument to convince most people it's really not worth saving 1 lousy MPG.

 

[mva]

mstrjon

Actually low tire pressure is more dangerous in terms of stopping distance, blowouts and hydroplaning.

http://www.officer.com/article/article.jsp?siteSection=19&id=27281

I don't advocate going over the maximum sidewall rating which is 44 psi on many modern car tires.

The best compromise on ride, handling and milage is likely the manufacturer recommended pressure. It is unsafe going much below the recommended tire pressures particularly in hot climates (see articles on Ford Explorer with Firestones for example).

If you don't mind sacrificing ride quality a bit and would like to improve your mpg, I believe you can safely go up from the recommended pressure as far as to the maximum rated pressure of the tire.

I drive to the road conditions and within my vehicles capability and follow at a safe distance.

 

[BuickGN]

You can't make a generic statement that covers all tires. My new set require 44psi to get a flat footprint. My old ones took 32. I go by footprint size to get my pressures. Trading 30' of stopping distance for a couple mpg is crazy.

 

[Mr Nice]

Capri,

Are you an engineer ?

You seem to know ALOT about tires.

 

[CapriRacer]

Originally Posted By: LT4 Vette
Capri,

Are you an engineer ?

You seem to know ALOT about tires.


Yup,

Been a tire engineer coming up on 40 years. I work for one of the majors, but when I first mentioned that I do a lot of posting on the 'net, some of the management expressed some reservations, so I decided not to mention the name on the paycheck.

In the big scheme of things, I don't think it really matters who I work for. Either what I post has meaning to them - or it doesn't.

 

[Kestas]

I just replaced all four tires in my Mercedes. I ran them from cradle-to-grave at 35 psi, 4-6 psi over recommended pressure. Rotated once. Inspection showed that each tire wore evenly across the treads.... no excessive center wear.

 

[kr_bitog]

I guess there are too many oversimplification on this thread, the right pressure also depends on the load rating of the tyre, sidewall connstructions, rim width size used, the load carried on vehicle, ambient temperature, tyre material, road surface and typical speed of the car driven.

That is true typically the manufacturer aims at the more comfortable ride with the recommended pressure, but too much additional pressure may not worth the MPG benefit with the risks of destroying the car suspension or the high speed wet traction.

 

[oilnoob425]

Originally Posted By: kr_bitog
That is true typically the manufacturer aims at the more comfortable ride with the recommended pressure, but too much additional pressure may not worth the MPG benefit with the risks of destroying the car suspension or the high speed wet traction.


No, it's not true. Manufacturer aims at number of factors including optimal traction. They are not as dumb as you think. The higher psi I tried was 6 psi over recomended 30. Car felt less stable; handling and traction diminished too. Driving on 44 or 50 psi would probably make car feels like it's on skates. No thank you. No MPG in the world is worth it. I find these type of threads ridiculous.