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Author Topic: Do higher tire pressures help mpg? Back to Topics
iaqxf
Rookie Author
Ohio

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Message Posted: May 19, 2011 6:29:26 AM


.............................................

tires at 10psi = 3.7% increase in consumption

tires at 30psi = 1.2% increase in consumption

Control, 35psi (manufacturer recomendation)

tires at 40psi = 6.2% decrease in consumption

tires at 60psi = 7.6% decrease in consumption

From Mythbusters test....


[Edited by: iaqxf at 5/19/2011 6:31:29 AM EST]
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HotRod10
Champion Author Wyoming

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Message Posted: Mar 26, 2015 10:45:28 AM

mybigtruck, if that 80 psi is within 5 or 10 psi of the recommended door panel pressures and below the maximum sidewall value, you shouldn't have any problems, especially on the front. On the rear, I suspect you will probably see slightly more wear in the center of the tread eventually, unless you keep it loaded with a camper or something most of the time.

I can see your reaction now, JimBlake56 - "but you just said...". A truck is a special case. The empty weight distribution skews heavily to the front, but fully loaded it is much heavier to the rear axle. Running empty, I keep the rear tires on my pickup right at the door panel and the fronts about 3 psi over. If I'm going to be hauling a heavy load, I bump the rears up close to the max sidewall pressure.
mybigtruck
Champion Author San Jose

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Message Posted: Mar 26, 2015 12:47:43 AM

i run 80psi on all 4 in my f350 truck
HotRod10
Champion Author Wyoming

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Message Posted: Mar 25, 2015 10:55:54 AM

"Leave the psi at what the tire recommends"

There is no recommended pressure on tires, there is only maximum pressure. The recommended pressure (for the OE tire size) is on the vehicle, typically on the driver's door panel. This value is the lowest allowable pressure, providing the greatest comfort for the passengers without sacrificing the integrity of the tires due to overheating.

Without sacrificing safety on either end, the best mileage will be achieved at the maximum sidewall pressure, the most comfortable ride is achieved at the door panel recommended pressure, and generally the best handling, cornering, and treadlife will occur between those two extremes.
Maintroll
Champion Author Lexington

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Message Posted: Mar 25, 2015 5:14:11 AM

Leave the psi at what the tire recommends, safety is involved here.
HotRod10
Champion Author Wyoming

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Message Posted: Mar 23, 2015 9:29:13 AM

"About 7 inches wide & 5 inches long."

And how much did it change by overinflating the tires by 3 psi?
JimBlake56
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Message Posted: Mar 23, 2015 8:03:40 AM

About 7 inches wide & 5 inches long. Not real straight edges, but the contact was more like an ellipse & less like a rounded rectangle.
HotRod10
Champion Author Wyoming

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Message Posted: Mar 17, 2015 6:52:47 PM

"That contact patch seems reasonable if you think about trying to slide 4 pieces of paper under your tire from 4 different directions. Roughly outlining a rectangle contact patch with rounded corners. I'll try to remember to do that to my tires."

Yeah, let us know how that turns out. I doubt very much the patch length is anything close to 4 inches.

"The circumferential belts control the circumference of the tire."

...and all sections of the tire - center and shoulders, controlling the circumference of the center of the tread to the same as the that of the shoulders, keeping the entire width of the tread in contact with the road.

poetdog73
Champion Author Toledo

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Message Posted: Mar 17, 2015 12:07:14 AM

whatever my tire recomends
JimBlake56
Veteran Author Akron

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Message Posted: Mar 16, 2015 4:37:38 PM

The circumferential belts control the circumference of the tire. They don't force it into a circle, but they flex at the road resulting in a slightly-larger circle with a flat spot at the bottom. The radial cords balloon outwards following the rounded shape of the sidewall. But that force distribution is resolved inside the tire structure and there's no external forces except the axle & the contact patch.

Using approximate but typical numbers... (my car)

3,400 pound car
60% forwards weight bias
215mm tire section width
38 psig inflation pressure

gives:
1020 pounds supported by each front tire (no dynamic loads included)
26.8 square inches of contact patch
6.8-inch-wide contact patch (90% of the section width)
4 to 4.5 inch long contact patch (depending how the corners are rounded)

That contact patch seems reasonable if you think about trying to slide 4 pieces of paper under your tire from 4 different directions. Roughly outlining a rectangle contact patch with rounded corners. I'll try to remember to do that to my tires.
cv
Champion Author Raleigh

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Message Posted: Mar 14, 2015 7:50:22 AM

I simply use the vehicle recommendations for both of mine.
HotRod10
Champion Author Wyoming

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Message Posted: Mar 13, 2015 2:39:31 PM

"That total upwards force against the tread of the tire, has to support the weight, because there's nothing else to support the weight."

Correct. However, the internal restraint will dictate **how** the force is transferred through the tread and the distribution of the force.

"I still think the total contact area has to follow F = P*A..."

Try putting some numbers to that formula and you'll find the contact area would have to be very large.

"with only a small contribution from the bending stiffness of the cords making up the tire."

It's not the bending stiffness of the cords, it's the tension stress of the **radial** cords (the ones that follow the circumference of the tire) limiting the diameter of the tire along the center of the tread to the same diameter as the shoulders, thus keeping the tread flat across the width of the tire.
JimBlake56
Veteran Author Akron

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Message Posted: Mar 13, 2015 1:45:46 PM

I wouldn't expect the width of the contact patch to get much smaller with increased pressure. I'd expect the length of the patch to be reduced. So a contact patch that's a bit like an oval, will shrink it's minor axis but not it's major axis.

And the forces can be applied as a free-body diagram. Downwards force of say 25% or 30% of the car's weight applied at the axle. Upwards (reaction) force of the ground against the tire. That total upwards force against the tread of the tire, has to support the weight, because there's nothing else to support the weight.

I still think the total contact area has to follow F = P*A with only a small contribution from the bending stiffness of the cords making up the tire.
drydem
Veteran Author Maryland

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Message Posted: Mar 13, 2015 8:05:55 AM

Tire deformation usually starts to occurs after the tire pressure exceed the maximum tire pressure stated on the tire side wall. Deformation occurs slightly so being off by a few psi isn't going to cause drastic lost of traction surface. As the tire pressure goes 10 psi or 15 psi over the maximum tire pressure as stated on the sidewall - there is a measurable lost in tire-to-road tread contact surface area so while rolling resistance is reduced measurably so is the tire's grip on the road and hence there is a greater safety concern/challenge. I've experimented with going over 10 psi over the maximum sidewall pressure rating on my Yokohama Avid s33D with my 2010 Prius - while there is a slight advantage for coasting in some driving scenarios - there are other driving techniques that have a better and higher real world benefit to cost/risk than going over the maximum sidewall tire pressure. In a close circuit track during a fuel-efficiency marathon setting the tire pressure 10 to 20 psi over the maximum sidewall tire pressure can provide a hypermiler an edge to achieve 100 mpg using certain types of vehicles at speeds less than 25 mph -- but for the real world - I would nix going over the max side wall tire pressure and focus my efforts on other ways to get the extra MPGs. In the real world, the driver has to drive much fasters, road surfaces are much rougher, and the vehicle must stop more frequently ... so rolling resistance is a smaller portion of the MPG equation... which makes the value of higher tire pressure less valuable...


[Edited by: drydem at 3/13/2015 8:11:46 AM EST]
Floridaman2013
Champion Author Florida

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Message Posted: Mar 13, 2015 7:38:52 AM

Always keep mine at 2 lbs below TIRE sidewall max. Best handling/fuel mileage/ tire wear.
Ratso
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Message Posted: Mar 13, 2015 7:08:59 AM

heh ...ok !
BlackArrowNY
Rookie Author New York

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Message Posted: Mar 12, 2015 9:58:00 PM

I have found that I consistently achieve better fuel economy by maintaining a tire pressure 2-3 psi higher than the sticker indication. I have not experienced more wear and handling is predictable.
HotRod10
Champion Author Wyoming

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Message Posted: Mar 12, 2015 9:13:15 PM

"Draw me a free-body diagram of the forces..."

You almost sucked me into that one, but a tire is not a free body. It has internal stresses and restraints. Of primary importance in the case of a radial tire is the radial belts, which restrain the middle of the tread to the same diameter as the edge of the tread. Want proof? Look across the top of the tire. Is it bulging in the middle or almost flat? A car tire is not a balloon; under pressure it's not shaped like one, and under load it doesn't behave like one.

As I suggested earlier, if you want real proof, put a piece of white paper down on a hard, flat surface and then drive over it with an overinflated tire. You'll find the same thing I did - the full width of the tread pattern is visible on the paper.

mrsteve68
Rookie Author Toronto

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Message Posted: Mar 12, 2015 3:24:29 PM

keeping the proper air pressure not only helps milage but also prevents premature wear on your tires and keeps you safer on the road
JimBlake56
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Message Posted: Mar 12, 2015 2:20:26 PM

With a tire not mounted on a rim, you can feel how limber the sidewalls are. Not enough force to support the car. Not even enough force to support the weight of a wheel, when it's not even bolted onto a car.

Draw me a free-body diagram of the forces showing how anything other than the air pressure is supporting the weight of the car?
HotRod10
Champion Author Wyoming

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Message Posted: Mar 12, 2015 12:54:44 PM

"Any increase in pressure will decrease the contact area."

I have to disagree. The pressure balance between the center and edges of the tread will change slightly, but with anything but the cheapest pieces of crap out there (I don't know if anything that flimsy is still allowed on the road - I haven't seen any in years), the tire has enough internal stiffness to keep the entire tread in contact with the road at a few psi over the door panel value. At 8-10 psi over, I would agree that losing contact area and the problems associated with that are very possible, and I would not recommend going that high.

"tire pressures several psi higher than the door-jamb numbers only make a moderate change in handling."

Agreed. My experience has been that 2-3 psi over results in slightly better cornering performance, but otherwise the handling was unaffected.

"I've seen longer life with more uniform treadwear by using pressures a couple psi higher than the door-jamb placard."

That's been my experience as well.
runner1256
Sophomore Author New Hampshire

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Message Posted: Mar 12, 2015 11:29:42 AM

I read the replies and learned quite a bit, thanks.. even though it wasn't my question :) I have been getting low gas mileage and am going to put more air in tires, try that.
GrumpyCat
Champion Author Alabama

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Message Posted: Mar 12, 2015 10:49:05 AM

Have to try it for yourself. If you can't measure it then it doesn't exist.

Measured the tread on 255/50-19 original equipment size tires on the SUV I purchased used and was surprised and disappointed to find the center of three tires were worn more than the edges. The 4th had less wear, and even, suggesting it was replaced early in the tire's lives.

Surprising as this is a 5,000 pound vehicle and 32 PSI is the nominal recommended tire pressure.

My 4700 pounder wears 245/45-19's at 42 PSI.
JimBlake56
Veteran Author Akron

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Message Posted: Mar 12, 2015 10:39:00 AM

Any increase in pressure will decrease the contact area. The tire still has to support the same weight, so higher pressure means less area. The contact area also changes shape which does things to the dynamic behavior.

Simple friction where the coefficient mu is constant is just a convenient simplification. It works pretty good in common experience like sliding a solid object across a table.

A rolling tire doesn't exactly work like that. Deformation of the tire & tread as the tire rolls allows the tire to "slip" sideways even without physical slipping of the individual tread blocks against the road. This gives you understeer or oversteer tendencies long before the tires actually begin sliding. That depends on under- or over- inflation.

Having said that, tire pressures several psi higher than the door-jamb numbers only make a moderate change in handling. Not the end of the world like some of these posts.

For the cost of tire replacement, I've seen longer life with more uniform treadwear by using pressures a couple psi higher than the door-jamb placard. But that also depends on driving habits.
Vin63
Champion Author San Bernardino

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Message Posted: Mar 12, 2015 9:46:45 AM

You have to weigh the cost savings with the cost of tire replacement.
HotRod10
Champion Author Wyoming

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Message Posted: Mar 10, 2015 10:48:25 PM

"I'm no scientist"

Obviously not. Even if tires were overinflated to the point of reducing the contact area, which would require pressure well beyond the maximum, it would not reduce the grip until the internal shear capacity of the rubber is exceeded (the tires are leaving rubber on the road). Traction is the product of force (weight) and the coefficient of friction, contact area is irrelevant under normal driving conditions on paved roads.

Modern tires hold their shape even at several psi over the door panel recommended pressure. If you want proof, lay a piece of white paper on a flat surface and drive over it with a seriously overinflated tire and see that the full tread pattern is visible.
Mininana
Champion Author New York

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Message Posted: Mar 10, 2015 5:14:05 PM

I'm no scientist but it seems to stand to reason that greater inflation means less road contact, and that sounds perilous...
HotRod10
Champion Author Wyoming

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Message Posted: Mar 10, 2015 4:22:31 PM

Of course higher pressures help mpg. A few psi over also increases tire life (less sidewall flexing and heat). It does make the ride a little rougher, but I haven't found it noticeable for my vehicles. I run all of mine about 3psi over the recommended on the door panel.

I don't like the ice and snow traction ratings for any of the low rolling resistance tires I've seen. I'll pay a little more in gas for better grip when I need it (which is fairly often around here).

[Edited by: HotRod10 at 3/10/2015 4:26:58 PM EST]
drydem
Veteran Author Maryland

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Message Posted: Mar 4, 2015 7:58:03 PM

btw: low rolling resistance tire do their magic with the roads are dry and smooth. If the roads surfaces are rough with debris or potholes, if the road is wet, icy, or covered with snow OR if roads are graded for traction like mountain side roadways THEN low rolling resistance tires and high tire pressures isn't going to help increase fuel efficiency.
drydem
Veteran Author Maryland

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Message Posted: Mar 4, 2015 7:44:59 PM

Higher tire pressure helps lowers the tire's rolling resistance which is the main energy cost for moving a truck forward under 20 mph and to move a car forward under 40 mph. Add low rolling resistance tires (like Goodyear Fuel Max or Bridgestone Ecopia )and a skilled hypermiling driver with higher tire pressure and this can help boost a vehicle's MPG by over 20% in the summer time. Usually any pressure setting within + or - 5 psi of the maximum tire pressure printed on the side wall of the tire should not cause the tire to prematurely wear or grow old. Most car manufactures recommend tire pressures that are 5 to 10 psi less than the maximum tire pressure on the side wall of the tire so that the tire absorbs some of the road shock/vibrations. As the tire pressure approaches the maximum tire pressure as stated on the sidewall of the tire - more and more road vibration/shockwaves will be transmittted from the tire into the car and steering column - making the ride much harsher when the road surfaces are rough ... As the speed of a truck goes over 35 mph and as the speed of a car goes over 55 mph - the major energy cost for moving forwaard becomes aerodynamic drag and value of low rolling resistant tires become less and less important to fuel usage.

I've done my research and verified the above concepts in the real world. and I've been able to get +70 mpg on a 2010 Prius in the summer time. I'm one of the few drivers listed in t he 3rd gen Prius 800 mile club and I'm in the top 40 for high MPG records in Priuschat.

[Edited by: drydem at 3/4/2015 7:52:27 PM EST]
Glasman
Champion Author South Carolina

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Message Posted: Mar 4, 2015 10:27:15 AM

yes, but the tires do not last very long..
JohnCur
All-Star Author New Jersey

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Message Posted: Mar 4, 2015 8:23:50 AM

yes
Mahalo5
All-Star Author Philadelphia

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Message Posted: Mar 4, 2015 7:51:24 AM

It may help mpg's but in the long run it shortens the life of the tire. That's what some car people have said. Who really knows.
iamjoekewl
Sophomore Author New York

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Message Posted: Mar 3, 2015 11:28:51 PM

yes
pt1KY
Champion Author Lexington

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Message Posted: Mar 3, 2015 4:51:30 PM

Yes
ShortyT2014
Sophomore Author Lexington

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Message Posted: Mar 3, 2015 12:02:16 PM

Yes
xmstr
Sophomore Author Michigan

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Message Posted: Mar 2, 2015 7:05:05 AM

No
GLM4205
Champion Author Toledo

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Message Posted: Mar 2, 2015 6:23:25 AM

Per manfacturer's recommendations.
hoopitup2000
Champion Author Virginia

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Message Posted: Mar 1, 2015 9:35:52 PM

yes
eccerr0r
Champion Author Fort Collins

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Message Posted: Mar 1, 2015 10:10:09 AM

Sure, let's trade off fuel economy for safety.
Then again, too low of pressure can also increase chance of tire failure too.
dontuknowOH
Champion Author Ohio

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Message Posted: Feb 28, 2015 12:36:30 AM

Yes, any lower pressure than what is proper tire pressure in all 4... tires, really wastes fuel mileage at any speed, even starting motion each time, also wastes when resuming back up to travel speed.

Any low inflated tire constantly hampers motion, by resisting rolling motion, almost similar to having a brake force turned on then creating a drag load. A visual check often is a cheap fix to save against worse fuel consumption. Air of course won't jump into the low tire just by looking at it.

Also if driving highway speeds over longer distances a low tire if mounted to the vehicles drive axle, that can cause extra stress/heat build up in the differential drive gears FWD or RWD, depends on where the low tire is in use. A 15---20# lower pressure tire, under inflated for example (very noticeable. Just do a visual check often, that saves $$$ later on!
ray44512
Veteran Author Ohio

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Message Posted: Feb 27, 2015 8:50:49 PM

Yes.
poetdog73
Champion Author Toledo

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Message Posted: Feb 26, 2015 10:15:02 AM

don't know
badbobKY
Champion Author Lexington

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Message Posted: Feb 26, 2015 10:05:19 AM

no
gvan
Champion Author Chicago

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Message Posted: Feb 26, 2015 9:21:53 AM

"Add a moderate increase of PSI for better edfficiency. Be sure to check the sidewall for info."

Be sure to check the owners manual or the placard on the driver door jamb for proper PSI....not the tire. A few pounds over the recommended pressure should not be a problem for handling or braking.
borego
Rookie Author Sarasota

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Message Posted: Feb 26, 2015 7:40:39 AM

depends on your tires and speed
dontuknowOH
Champion Author Ohio

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Message Posted: Feb 26, 2015 5:46:50 AM

I just try to stay within 2-3# of the max rating on the tire, running good tires. No problems yet on any of Our vehicles, past/present! Better MPG's!
nru
Champion Author Twin Cities

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Message Posted: Feb 25, 2015 10:49:33 PM

Better mileage - longer stopping distance and less steering control
gas2guzz
Sophomore Author New Jersey

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Message Posted: Feb 25, 2015 7:47:58 PM

slightly
PaylessKY
Champion Author Kentucky

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Message Posted: Feb 25, 2015 2:54:04 PM

Use the size tires your vehicle came with, and check the tire placard for proper psi.
cheapmonkee
Champion Author Portland

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Message Posted: Feb 25, 2015 2:13:05 PM

Add a moderate increase of PSI for better edfficiency. Be sure to check the sidewall for info.
jameskel
Rookie Author Washington

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Message Posted: Feb 18, 2015 5:03:14 PM

I always go with what the TIRE manufacturer says on the tire, not what the car maker wrote in its manual 20 years (and 8 sets of tires) ago.

James
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