How much of an increase in mpg can I expect with overinflated tires?

[theclencher]

Originally Posted by Ernie Rogers

Let's try some numbers. A 195 wide tire is 195 /25.4 = 7.68 inches wide at the tread. The load on one tire is about 900 pounds. At 40 psi, the area of the contact patch is--

A = 900 lb /40 psi = 22.5 square inches.

Divide that by the tread width to get the other dimension--

22.5 /7.68 = 2.93 inches long.

To make 2.93 inches of curve flat on a 25 inch diameter tire, you have to deflect the center of it by 0.11 inches.

The harder it is to get the tread surface to move in that amount, the more energy the car loses as it rolls. That means soft rubber is better, and a thin tire is better. "Low friction" rubber and tire construction is also better, and that's what you are paying for in an "energy tire."

Ernie Rogers

A 195- tire is not 195 mm wide at the tread- the designation is for max sidewall width (section width). That throws all yer numbers off but the rest is good I think... xept the phrasiology of "the harder it is to get the tread surface to move..." isn't quite right, as overinflating will do that yet lower r.r.. More accurately, if psi is ignored and we consider how much energy the rubber in the tire structure absorbs (hysteresis) THAT is what is soaking up the energy. That is where rib treads are better than tread blocks (less flexing), thinner is better than thick (less energy absorbtion), worn out is better than new (less flexing), etc.

http://www.tirerack.com/tires/tirete...e.jsp?techid=7

P.S. A little something on sidewall psi ratings:

"It is important to note that the maximum load is rated at an industry specified tire inflation pressure that is often lower then the tire's absolute maximum tire pressure. The tire pressures used to determine the maximum load the tire is rated to carry is based on the sizing system industry standards applied to the tire.

Sizing System Tire Load Range Load Pressure
P-metric Light Load
Standard Load
Extra Load 35 psi
35 psi
41 psi
Euro-metric Standard
Reinforced or Extra Load
36 psi
42 psi


However, the tire's maximum inflation pressure may be greater, such as 300 kPa (44 psi) in this example or even 350 kPa (51 psi). This is done to accommodate the vehicle manufacturers desire to tune the tires' high-speed capability, handling qualities and/or rolling resistance to better suit the vehicle."

http://www.tirerack.com/tires/tirete....jsp?techid=21

Looksta me like there is plenty of latitude for interpreting sidewall "max psi"

[trebuchet03]

Originally Posted by Ernie Rogers

Yes, "inflation stiffness" is greater.

When you push down on an uninflated wheel and tire, it won't even support your body weight-- you can neglect the stiffness of the rubber in determining the area of the contact patch, so--

F (load on the tire) = P (tire pressure) x A (contact patch area)

Let's try some numbers. A 195 wide tire is 195 /25.4 = 7.68 inches wide at the tread. The load on one tire is about 900 pounds. At 40 psi, the area of the contact patch is--

A = 900 lb /40 psi = 22.5 square inches.

Divide that by the tread width to get the other dimension--

22.5 /7.68 = 2.93 inches long.

To make 2.93 inches of curve flat on a 25 inch diameter tire, you have to deflect the center of it by 0.11 inches.

The harder it is to get the tread surface to move in that amount, the more energy the car loses as it rolls. That means soft rubber is better, and a thin tire is better. "Low friction" rubber and tire construction is also better, and that's what you are paying for in an "energy tire."

Ernie Rogers

I'm still having a problem from an intuitive stand point.... Low profile tires are associated with a bumpier ride.... Wouldn't that indicate that the tires are not deflecting as much compared to their traditional aspect ratio tires?

EDIT: ^^ Okay, so I figured that part out -- that's a dynamic situation, we're talking statics...

In any case... here's what I found from internet searching....

Low Profile for trucks claimed 5% lower RR... But trucking tires are likely a whole different ball game...
http://www.conti-online.com/generato...rofile_en.html

New LP tires v. LP when new ---- again, for trucks

When low profile tires first came out, they were about 4 percent more fuel efficient. Low profile sidewalls are shorter, and therefore stiffer. That reduces flex but only slightly. Smaller tires weigh less than larger tires, so less energy is needed to get them rolling, and also to stop them. The weight advantage only comes into play if you cube-out. If the weight saved on the tire is added to the load, any fuel savings disappear. The added stiffness of low profile tires is no longer measurable, due to improved tire engineering. It is safe to say that your choice between 22.5 or 24.5 wheels, and 11 or metric sizes should be based more on load and speed requirements than on fuel economy. However, since low profile tires on 22.5-inch rims do lower the chassis by about an inch or so, they do reduce under-chassis turbulence slightly, but the difference is barely measurable. It is probably not worth switching to smaller tires, since any gains in fuel economy would be offset by the added cost of re-gearing to keep your engine operating in its most efficient rpm range. Remember, power and economy are affected by transmission, drive axle ratios and tire size (revolution per mile). Change one, and you throw the equation off. With today's new tires, there is virtually no difference in mpg due to tire size.

It's interesting that while searching... There was quite a bit of information on trucking and LP tires - information that, at the very least, seemed credible... For automotive applications... it's all over the place with claims

So, deflection energy makes sense to me right now... We'll see when I wake up tomorrow <--I should really stop including anecdotal "evidence" in my assumptions

[skewbe]

Originally Posted by bowtieguy

brilliant skewbe! just inflated mine to 60psi(from 44) and i'm gonna try that. thank you.

Originally Posted by trebuchet03

ha, that would totally work too.

I should qualify this a bit. If you have been driving around with "underinflated" tires, the outsides WILL have worn down and pumping them up can give the impression that there is a bulge in the middle.

I think the chalk test followed by rolling forward a couple tire revolutions would be better than paint for identifying "bulge". The paint might be better for monitoring the effect of alignment changes in "the real world" (i.e. cornering/accelerating/braking).

Originally Posted by trebuchet03

...Actually, that's one method to check gear lash on really really big gears -- spray on some paint and then run the machine for awhile... Then check how the gears are meshing.

Ah yes, reminds me of a little tube of prussian blue I've had for years.

[garyhgaryh]

Here's a reason not to run too much pressure, granted I filled my tires to 40psi and the tires are rated to 35psi.

http://www.gassavers.org/showthread....&highlight=age
Gary

[jandree22]

yeah I'll concede that when searching around after I posted what I did above, I see that playing with sidewall max isn't a prescription for disaster as I previously assumed. Particularly with Prius owners, those that used OEM specs had their outer edges wear out super fast because of increase heat/flexing, and those that used sidewall max had minimal wear, even including the middle of the tire.

And yeah, brucepick, your points about load capacity vs. psi make perfect sense

[skewbe]

Originally Posted by garyhgaryh

Here's a reason not to run too much pressure...

Not exactly, it happens to "normally" inflated tires too. Given that kickflipjr doesn't know the history of the tire in question, it may also be that an abused tire (pot holes, underinflation, manufacturing defect) is less suitable for over inflation, but it would also be less suitable for driving on in general, and one incidence does not a study make.

I love how lug nut fixed his with rubber cement His failed at 32 psi.

[trebuchet03]

Originally Posted by skewbe

Not exactly, it happens to "normally" inflated tires too. Given that kickflipjr doesn't know the history of the tire in question, it may also be that an abused tire (pot holes, underinflation, manufacturing defect) is less suitable for over inflation, but it would also be less suitable for driving on in general, and one incidence does not a study make.

I love how lug nut fixed his with rubber cement His failed at 32 psi.

He did say

It was probably the combination of old cheap tires and me hitting a bump or pothole hard.

And we keep saying, don't over inflate old tires For that matter - don't even use old tires....

[omgwtfbyobbq]

Originally Posted by skewbe

Not exactly, it happens to "normally" inflated tires too.

X2. I hate an entire set of Contis fail. But I think it had more to do w/ my mom leaving 'em out for days on asphalt during summer since I never overinflated those. For that matter, I had decade+ old tires fail after I overinflated 'em 5psi... But I don't think the fault was because of overinflation as opposed to them being about half my age.

[bowtieguy]

skewbe,

thank you for the tip. tried it (paint across the tread) and found that 60psi is too much for the rears, but not for the fronts. weight distribution right?

[skewbe]

Wow, 60 huh? Braver than me Weight distribution could be a factor in the difference.