Tire Roll out

[LiveWire]

The discussion of tire roll out in another thread prompted this one. I am fishing and know this could possibly start a heated thread so I made it a new one.

On another board, it was being discussed about ways to save mileage. One of the ones commonly suggested (even by George Bush) is to raise tire pressure. It was then brought up by several people who had done real world testing that they saw no improvement in mileage running the max tire pressure over the vehicle sticker's pressure. The response from other's was that the reason no increase was seen was that the tires were now bigger and the odometer was off. Something commonly brought up is the term rolling radius. The definition of that is often given is the distance from the center of the wheel to the ground. Wikipedia's truth by consensus sides with the aforementioned arguments. I am in the minority that say this is not correct. I concede the following points: Roll out is less than the circumference of a tire. Roll out is often very close to calculating the circumference from rolling radius as measured above.

My assertion is that if you raise your tire pressure from the vehicle sticker to the max pressure, your tire roll out will not change. Assuming I am in the minority here as well, fire away. I'll throw in other statements that may spark some thought. If you install wider tires, the optimal air pressure would now be lower than the vehicle sticker. I run the rear tire pressure in my truck lower than the sticker unless I am hauling a load.

[SPEC]

Get your pressure guage and tire tape out

[SPEC]

Bias or radial carcass

[Dunebound69]

I have seen low tire pressure produce lower mileage. I picked up 2 mpg on my Olds Aurora by maxing the tires on a 800 mile trip. Mileage figured by paper not the computer. Increased air pressure would change the rolling radius. Lets consider a flat tire, its rolling radius is shorter than a inflated tire. Granted that is an extreme. But is a visual of the concept. If max air pressure changes the bulge in the side wall it has effected the rolling radius. The effect of the air pressure will be varied by tire manufacture, load rating, vehicle weight, load, and tire size. A narrow taller tire produces less rolling resistance than a short wide tire. I do fear this will produce a little spirited conversation. As the variables here are substantial.

[fabr]

Big,big can of worms here for sure including safety issues.

[Boostinjdm]

The tire pressure on the vehicle sticker only applies to original equipment.  If you run different tires than what the vehicle was originally equiped with, then the sticker is not correct.  I always go by what the tire says.

[Boostinjdm]

Upping the pressure also has limited effects because a taller tire takes more power to accelerate, more power usually means more fuel.

[fabr]

The tire pressure on the vehicle sticker only applies to original equipment.  If you run different tires than what the vehicle was originally equiped with, then the sticker is not correct.  I always go by what the tire says.

Really? How does the tire know what load it is carrying? Tire air pressure is specified on OEM tires by the vehicle manufacturer(in a perfect world anyway) according to the load it needs to support to maintain a full ,evenly loaded,contact patch with the road.  That is what a tire is designed to do in the first place and the only way that happens is with vehicle appropriate air pressure.That is what we should try to do as we carry differing loads. I run 50 pounds in my duals empty and the full 80 when trailering with the big trailers and somewhere in between when using any of the smaller ones. I pull from zero payload to 20000+ pounds. I do a lot of miles a year. I always have even wear across the tread.

[Boostinjdm]

Really? How does the tire vehicle know what load tire it is carrying wearing?

Also, since you brought up towing.  I have yet to see a sticker with a chart on it saying what pressure to run for xxxx lbs of cargo or tongue weight.

From my experience, people don't by oem replacement tires either.  They buy what's on sale or what looks good.  Many don't even pay attention to dia.

[fabr]

I bow to your vast experience.

[Nutz4sand]

I myself agree that one should use the numbers on the side of the tire. Not the door sticker.

If you have a vehicle thats even only a few years old but NEW tires with new technology what was specified may not apply at all due to newer tire tech. The newer tires tech may be so different that they could not have guessed when the vehicle was built.

I have also seen a lot of newer tires that run higher pressures. Running the door sticker of 32 lbs. (this was a little older car) made the tires nearly flat as they were about ten years newer tech and required 44 psi. At 32 they were flat just by looking at them. It was certainly not safe to drive.

There aint no one right answer as to many things can be mixed in from comfort tires to long life tires to performance to snow tires and on and on.

But I still believe the side of the new tire supercedes the old door sticker.

If things dd not get any better your vehicle would have round rocks for tires. 

[chrishallett83]

My interpretation of the intent of all this 'higher tyre pressure reduces fuel consumption' talk from government departments is that they are trying to get people to at least check their pressures and maintain them more regularly. Guys on this forum and other like-minded fellows will do a much more thorough job of maintaining tyres ad tyre pressure than the "average joe".

While over-inflating a tyre past the recommended levels for its fitment will have little effect on fuel mileage, running a tyre under-inflated can make a big difference. As most "average joe"s pretty much never check their tyres, most cars are running around on quite soft tyres. Not only does it accelerate wear (wasteful), if half of the cars in America are using 10% more fuel than they should, just inflating all the tyres to the correct level could reduce Americas passenger car fleet fuel usage better than taking hundreds of thousands of cars off the road altogether.

Not to mention the safety aspects of so many cars driving around on under-inflated tyres...

[fabr]

I agree chris about underinflation but am completely dismayed that no one has mentioned the safety hazard of overinflation.  For example: you have a 1/2 ton pickup and you put on 40" +  wideass tires on it. Do you run the tires a max inflation? What occurs if you do? You run on a rounded  footprint that will wear out the center of the tread. At the extreme you are running on a very narrow ridge that is very unsafe. You will need to run considerably lower pressure to prevent it. Info printed on the tire specs pressures at max pressure/max load. If you are not loading the tire to it's max design load then max pressure is WRONG and very possibly unsafe due to the very narrow contact patch due to the over curvature of the tread in relation to the road. The only proper inflation pressure is the pressure needed to maintain the tires design profile and provide even wear across the tread face. Anything more or less is wrong. Don't believe it? Argue with the tire manufactures.

[Nutz4sand]

Something I noticed that bothers me in real life is WHY the tires tend to leak.

The biggest reason I have dealt with is when I take my tires in to be worked on. Half the time even if you ASK they will not clean the rims bead. So the tires tend to slowly leak there.

My old autoshop teacher in High School was always a little irate that tires being as important as they are were the one thing NOT needing a state certificated license to work on! Any kid off the street could be trained in ten minutes and let go at it! Rediculous!

There is a tire place near me and the last time I had a tire fixed there and older genntleman took the rim and spun it on the tire machine while cleaning the bead with a air powered wire wheel. He then used a rag and something to clean the tires bead before he applied the slippery stuff to get it to mount easy. THAT tire NEVER needs air. But the others I have had worked on at other places always do! When I get new tires if I am still in this are I am going there and if the guy aint there that day I will either go back or ask that they all do that there.

He made me a repeat customer.

But shops that say eff it and just pop the tire on dirty are as big a reason to fault as anyone. If the tires never leaked (unless you run over something that puts a hole in the outside carcass) the cars on the road would be getting better mileage all along AND be safer han a car with a low low tire and a driver who cannot tell or feel the differeance!

I guess its credit where credit is due. It what I see as a bigger part of the problem. If tires never leaked till you ran something over...

If I was fortuneate enoguh I would buy me a tire machine and a balancer and do them myself and for friends and family. I used to do all my tires and rims when I had access to said amchines and I never had a leaker as long as the tire itself was sound.

Plus many people I watch balance tires get the location from the machine and then spin the tire while leaning over it and put the weight in the wrong place. A little pet peave of mine. But thats another thread maybe? 

[SPEC]

There are some other factors that haven't been mentioned here yet...
Rim width VS. tread face width
Radials VS. bias ply Lots of bias ply buggy tires out there...
Tire temp...
Up here when it's -50 F I see all kinds of people on the side of the road with tires off the rim...I used to pull over and help them
Tread compound ...Many performance tires are really soft and wear quickly, A grippy tire will not give the fuel milage or tread life that a harder one does
Chris and Nutz had some really good points too