14mpg!

[Boaz]

I strongly advise against compensating for poor fuel mileage by increasing tire pressure beyond specifications.

There is a correct tire pressure specification for every vehicle/tire combination. This "correct tire pressure" ensures that the tire has the correct profile for the weight (vehicle/axle) that it is supposed to carry. Both under and over inflated tires can be unsafe and cause additional wear and tear on the vehicle and tires.

One thing to note - the maximum tire pressure rating on the sidewall of the tire is for the maximum tire pressure required for the tire to bear it's maximum load rating. This is not the same thing as the recommended tire pressure for your vehicle. Your vehicle, even under load, may not weigh enough to even come close to the maximum load rating of the tire. Case in point - my Ranger has 31.5x10.5x15 floatation tires. These tires work just fine on full size pickups. Obviously the Ranger will never be as heavy (loaded or unloaded) as a full size pickup. Hence, my recommended tire pressure is well below the maximum tire pressure rating on the sidewall of the tire. If I exceed the recommended tire pressure, then my Ranger is no longer heavy enough to create the correct tire profile.

You should make sure that the tires have the correct pressure for the vehicle on which they are mounted. You can fine this information on the door sticker or in the owner’s manual.

If you should go to a non specified OE tire size, then you should ask the tire installer (or call the tire manufacturer) what the recommended tire pressure should be for your tire and vehicle combination.

Get an accurate pressure gauge and make sure you follow these recommendations. Check your tire pressure at least monthly. In between tire pressure checks visually inspect your tires daily. Here's what to look for.

Over inflation profile:

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Proper inflation profile:

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Under inflation profile:

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You can also use chalk to verify that you have the right tire pressure. Draw a line across the surface of the tire. Roll the vehicle in straight line so that the tire turns a couple of revolutions. Check the chalk marks. If the chalk marks is worn off all the way across the tire surface then you have the right tire pressure. If your tires are over inflated then your results will probably look something like this:

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If your tires are under inflated they will look something like this:

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Over inflated tires reduce the tires contact patch with the road and therefore reduces traction for turning, braking, inclement weather etc. Over inflated tires can cause serious steering control problems especially at higher speeds. Over inflated tires wear out faster and cause more wear and tear on the vehicles suspension parts because they can't absorb the shocks as well from abnormalities in the road (pot holes etc.).

Under inflated tires are said to be the leading cause of tire failure. Under inflation increases rolling resistance and heat. There is less "air" to support the load (overloaded tire) and the sidewalls have to make up for it. The increased sidewall flexing and generated heat causes damage to the tire and can cause blowouts and tread separation. Obviously the increase friction and heat reduces fuel mileage. The sidewall strength is reduced and can cause handling problems. Tire contact patch is also reduced - further compromising the handling and traction characteristics of the tire.

Bottom line - make sure your tires are set at the "correct" tire pressure for your vehicle. Don't try to out smart the engineers who designed the thing (most people are not qualified to do so.)

There are other things you can look at besides tire pressure to improve fuel mileage. Make sure air filters are changed and other periodic maintenance is done. Make sure that your engine/transmission controls have the latest updated flashes. Another thing to check are your driving habits.

When all prudent measures (fooling with tire pressure not being one of them) have been done to correct fuel mileage, then there comes a point where you are just going to have to learn to live with what you got. You can only get so much efficiency out of an engine/transmission/vehicle combination. If this combination doesn't meet with your satisfaction and it's a significant enough issue for you, than perhaps a new/different vehicle is in order.

I suggested running the tires at 38-39 psi.. spec is 35psi..... I am no where near the maximum tire pressure on the sidewall.... whats wrong with that?

BTW thanks for the pics and article. I will try the chalk test to see where I am on tread contact path.

 

[T^2]

I suggested running the tires at 38-39 psi.. spec is 35psi..... I am no where near the maximum tire pressure on the sidewall.... whats wrong with that?

BTW thanks for the pics and article. I will try the chalk test to see where I am on tread contact path.

Well, I'm not trying to attack anybody. I'm just providing a public service/safety announcement - so to speak.

A couple of pounds per square inch - plus or minus - can make a big difference, especially depending on vehicle and tire combinations. That's why they recommend that you get an "accurate" tire pressure gauge. Three to four pounds above specification can definitely start to round off the profile of the tire and cause the problems that I mentioned above.

I'm a believer in running tires the way that they are "designed" to run. I'm an engineer, so you can see why I might be big into that.

As far as I'm concerned, there is only one tire pressure to run your tires at - and that's the "correct" tire pressure.

Deflating tires for off road use may be another story - maybe not. However, here we are talking about on road use and maximizing fuel mileage. Playing with tire pressure, in my opinion, is not the way to improving your MPG numbers.

 

[grogiefrog]

I suggested running the tires at 38-39 psi.. spec is 35psi..... I am no where near the maximum tire pressure on the sidewall.... whats wrong with that?

BTW thanks for the pics and article. I will try the chalk test to see where I am on tread contact path.

Well, I'm not trying to attack anybody. I'm just providing a public service/safety announcement - so to speak.

A couple of pounds per square inch - plus or minus - can make a big difference, especially depending on vehicle and tire combinations. That's why they recommend that you get an "accurate" tire pressure gauge. Three to four pounds above specification can definitely start to round off the profile of the tire and cause the problems that I mentioned above.

I'm a believer in running tires the way that they are "designed" to run. I'm an engineer, so you can see why I might be big into that.

As far as I'm concerned, there is only one tire pressure to run your tires at - and that's the "correct" tire pressure.

Deflating tires for off road use may be another story - maybe not. However, here we are talking about on road use and maximizing fuel mileage. Playing with tire pressure, in my opinion, is not the way to improving your MPG numbers.

T2, thanks for the write up. I'm going to try the chalk test as I know I'm a little high...

 

[2003KJ]

I'm going to try the chalk test as I know I'm a little high...

I hope you mean your tire pressure!!!!! [-X

LOL!!! Sorry grog, couldn't pass that one up \/

 

[Fiasco]

sniffing chalk? Is that what the kids are up to these days

 

[grogiefrog]

I'm going to try the chalk test as I know I'm a little high...

I hope you mean your tire pressure!!!!! [-X

LOL!!! Sorry grog, couldn't pass that one up \/

#-o #-o #-o #-o #-o #-o

I set myself up for that one, lol!

#-o #-o #-o #-o #-o #-o

 

[fujitsubo]

Tire Pressure

While I haven't been able to track my mpg yet, I noticed the Jeep is quite thirsty on gas and that when i was running at 30 psi, the tires slipped quite a bit on wet pavement. What I mean by slipping is accelerating from a start on a slight hill or turning right on a red light, and that's not gassing it either. On dry pavement, I noticed that if I corner a little harder than normal, the tires were squelching a bit. [-( Not impressed at all. I bumped the psi on all tires to 38 and see if that will help. I will invest in a new filter as well as it seems dirty at 17,000 kms.

rob

 

[T^2]

Re: Tire Pressure

While I haven't been able to track my mpg yet, I noticed the Jeep is quite thirsty on gas and that when i was running at 30 psi, the tires slipped quite a bit on wet pavement. What I mean by slipping is accelerating from a start on a slight hill or turning right on a red light, and that's not gassing it either. On dry pavement, I noticed that if I corner a little harder than normal, the tires were squelching a bit. [-( Not impressed at all. I bumped the psi on all tires to 38 and see if that will help. I will invest in a new filter as well as it seems dirty at 17,000 kms.

rob

"Under-inflated tires on your vehicle lead to poor or delayed braking, steering and acceleration. Under-inflated tires may squeal when stopping or cornering even at moderate speeds, particularly on warm pavement."

Be Tire Smart

http://www.betiresmart.ca/inflation/proper.asp?loc1=inflation&loc2=proper

A previous post stated that spec was 35 psi. Have you tried that?

 

[grogiefrog]

I tried the chalk test this evening (Note to Tim, on my KJ!).
(Alcy)
The chalk line was originally consistent in the photo below.

My stock tires are running at 39 (door says 34). Looks like even wear. When I first got the KJ, it seemed that last summer into fall they were loosing a pound per month. So I then set them a little high to 37 over the winter. I haven't touched them since and now this summer their up a little to 39 (hot weather).

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[Trodo]

Grogiefrog, what size tires do you have?

 

[JeepJunky33]

Fiasco I too have a 2003 KJ Renegade. But my hwy was always around 16mpg or so. and city was around 12-13mpg. I got the vornado air tornado thing. It's not a gimmick, it really does work. My mpg jumped to around 17-18mpg on the hwy. and then I got a K&N Cold Air Intake. Aside from it making the intake sound great my Hwy mpg jumped again to around 18-19 on a cruise control 20mpg. Either way I would recoment and vornado it costs about $60 and could not be eisier to install. It took about 3-5 minutes. It is just a metal fin thing you put in the intake tube that spins the air. (I know what your all saying, but It really did work) If you would like contact me (AIM SGmad) and I will sell you mine cause I dont use it anymore so it is sitting in the box.

 

[grogiefrog]

Grogiefrog, what size tires do you have?

Just the stock Goodyears (235/70).

 

[Trodo]

Cool, now I feel a little bit safer in bumping up the pressure from the results of your chalk. I have been running at 35 psi.

 

[T^2]

I tried the chalk test this evening (Note to Tim, on my KJ!).
(Alcy)
The chalk line was originally consistent in the photo below.

My stock tires are running at 39 (door says 34). Looks like even wear. When I first got the KJ, it seemed that last summer into fall they were loosing a pound per month. So I then set them a little high to 37 over the winter. I haven't touched them since and now this summer their up a little to 39 (hot weather).

You must be registered for see images

I suggested the chalk test as a visual way to make sure, or reaffirm, that your tire pressure was "in the ballpark" of where it was supposed to be. When I say ballpark I mean that you are not way off enough in either extreme that tire would start to become distorted enough to produce the result shown in the above pictures. I suspect that you could be off +/- several pounds (depending on tire/vehicle combination) from the optimum tire pressure and still get chalk test results that look close to what you want. So, I wouldn't substitute the chalk test for an accurate pressure gauge (and known recommended tire pressure).

It might be interesting if you set your tires at the recommended 34 psi and did the chalk test again. It might also be helpful to roll the tires more revolutions until the chalk marks start clearly disappearing. If you should do this and snap another picture, then draw another chalk like across the tire after the test. This step could be useful for comparison purposes when viewing the picture.

I will say this, your tire looks like it may be wearing evenly across it entire surface - which is good. However, it's really hard to tell because one can't see the 3 dimensional tread depth from a 2 dimensional photograph (especially at this angle).

Chalk test can be helpful in certain situations such as when non OE tire sizes or tires with different load rating are used. In these cases the recommended pressure may or may not be appropriate. If not appropriate, then this visual test can at least tell you that you are not "way off" from where you ought to be.

When running on tires that are OE spec (or close to it) I still recommend not deviating very far from the manufacturer recommend tire pressure for the vehicle. I also advise ignoring conventional wisdoms about lowering tire pressure for snow etc.

Granted this is not my field of engineering, but at least let me suggest this - these number where most likely not pulled out of a hat. Safety and performance where no doubt the driving forces behind their genesis. If one should feel that they are more qualified to come up with the right numbers then all I can say is - more power to you. As for me, I'll stick with the sticker on my door.

 

[grogiefrog]

T2, that's cool. Your information is good. I was just giving it a try. I did try the other front tire first and ran it about half way down my street to where the chalk was 90% gone. So I then did the opposite tire and ran it just enough for the photo above.

As said, I am just at 39 (or so) due to last adding air when it was cold out.

I don't think that I have noticeably seen an increase in MPG as a result of riding with more air in the tires. But who knows? MPG from a brick can easily be influenced.

Also, when the tires are at 34, it looks like to me that they look low, due to a bulge. But I'm not a tire expert, lol.

 

[Trodo]

This is intresting, because opinion varys greatly when it comes to how much tire pressure you use for the 235/70/16's because on the door it lists 15's as 33 psi. Mine, doesn't list 16's.
Almost everyone here, gets the 16's when they buy their Liberties.

Does anyone know what the 16's pressure is supposed to be, meaning what the factory recommends, not the reading on the side of the tire.

 

[Boaz]

This is intresting, because opinion varys greatly when it comes to how much tire pressure you use for the 235/70/16's because on the door it lists 15's as 33 psi. Mine, doesn't list 16's.
Almost everyone here, gets the 16's when they buy their Liberties.

Does anyone know what the 16's pressure is supposed to be, meaning what the factory recommends, not the reading on the side of the tire.

Trodo, do you visit T4R.ORG? That avatar looks familar....

 

[Trodo]

T4R.org doesn't sound familiar. I just thought the cat with a lime helmet was too damn funny. I keep trying to find one of those mini helmets that 31 flavors used to put ice cream in for my wife's cat, it's sort of ********.

 

[fujitsubo]

mpg

Just did some math and I figure I'm getting 19.8mpg on my 2004 Liberty. That sounds great for some reason yet it's still hitting me at the pump and it seems thirstier than that.

rob

 

[Marlon_JB2]

That sounds like a great number.

Took a trip yesterday. 80 miles to, 80 miles from. Going to my destination, I averaged 24MPG. Coming back, 22MPG. Both are great numbers, especially at 75MPH. (EVIC calculations... even if they are wrong, both are good numbers, if 10% lower) However, my EVIC tends to be 90% accurate.