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  #13  
Old 12-12-2013, 05:58 AM
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Quote:
Originally Posted by mswlogo View Post
I'm running OEM 18's which recommends 36 lbs and I'm running 40 lbs.

I think it adds about 4 MPG on average. With a very light foot. If you are heavy footed you might notice less.

What do the 20's recommend for pressure?

I wonder if the 20's vary pressure with temp changes a lot less because they have a lot less air volume. Or should it be the same in theory? I'm not sure

I figure an 18" wheel could be inflated quite bit before it reach the harder feel of the 20".
There is no way adding 4 extra psi of air is giving you a 4 mpg increase. That would mean your getting the rated highway mileage for a average mpg. That in itself isn't impossible but getting it from 4 psi is.

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  #14  
Old 12-12-2013, 07:21 AM
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Re: Recommended Tire Pressures

Quote:
Originally Posted by mswlogo View Post
I'm running OEM 18's which recommends 36 lbs and I'm running 40 lbs.

I think it adds about 4 MPG on average. With a very light foot. If you are heavy footed you might notice less.

What do the 20's recommend for pressure?

I wonder if the 20's vary pressure with temp changes a lot less because they have a lot less air volume. Or should it be the same in theory? I'm not sure

I figure an 18" wheel could be inflated quite bit before it reach the harder feel of the 20".

Wheel size has little if any effect on PSI. I highly doubt that over inflating your tires gets you 4 mpg better mileage. My 18" OEM's recommend 33 psi and I run mine at 35 year round. If you over inflate you will wear out the tire center and have less traction and lower level of control due to a smaller contact patch. You will also have a harder and rougher ride. Tire type and size will effect this also.
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  #15  
Old 12-12-2013, 07:54 AM
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Re: Recommended Tire Pressures

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Originally Posted by peckmv View Post
Wheel size has little if any effect on PSI. I highly doubt that over inflating your tires gets you 4 mpg better mileage. My 18" OEM's recommend 33 psi and I run mine at 35 year round. If you over inflate you will wear out the tire center and have less traction and lower level of control due to a smaller contact patch. You will also have a harder and rougher ride. Tire type and size will effect this also.
Got 75k miles out of my last Jeep tires over inflated a few lbs. Have 45k on my wifes.

I'd get 20-21 mpg on my city driving commute with 36lbs. I get 24-25 if I put 40 lbs.

My wife goes from low 30's to high 30's easy on a 50 miles mostly highway commute.

So where is the rule book that says 2lbs over inflation won't wear out center but 4lbs will?
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  #16  
Old 12-12-2013, 08:01 AM
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Re: Recommended Tire Pressures

Quote:
Originally Posted by loveracing1988 View Post
There is no way adding 4 extra psi of air is giving you a 4 mpg increase. That would mean your getting the rated highway mileage for a average mpg. That in itself isn't impossible but getting it from 4 psi is.
It used to be epa rating is based on a loaded vehicle.

4 adults and luggage.

My wife gets 38 mpg easy, epa highway is 31.

How you drive is huge. Lots of coasting. Lower tire ppressure kills the coasting.

My wifes is a stick.
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  #17  
Old 12-12-2013, 08:13 AM
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Re: Recommended Tire Pressures

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Originally Posted by mswlogo View Post
So where is the rule book that says 2lbs over inflation won't wear out center but 4lbs will?
It's the same book that dictates you should change your oil every 1,000 miles or 10,000 miles. And in both cases the answer is probably somewhere in the middle.

Higher air pressure will tend to wear tire centers. Low pressure will wear the edges. Tire companies have tested and proved this over time.

But, the exact 'proper' pressure is a little fuzzy. The tire has the 'maximum' pressure stamped on it. While this might be a conservative #, I would pay some attention to it. The auto maker gives a 'recommended' pressure. The auto maker needs to maximize mileage and handling while still keeping an eye on smooth ride.

When radials first became popular, many luxury manufactures were recommending 23 PSI. That gave radials a reputation for blowing out. But that Coupe de Ville was smooooth!

There is no perfect PSI, just don't ignore the guide lines.
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  #18  
Old 12-12-2013, 08:26 AM
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Re: Recommended Tire Pressures

Quote:
Originally Posted by mswlogo View Post
Got 75k miles out of my last Jeep tires over inflated a few lbs. Have 45k on my wifes.

I'd get 20-21 mpg on my city driving commute with 36lbs. I get 24-25 if I put 40 lbs.

My wife goes from low 30's to high 30's easy on a 50 miles mostly highway commute.

So where is the rule book that says 2lbs over inflation won't wear out center but 4lbs will?
No rule book, but it has been well documented what high pressures do to your tires and performance. I really don't care what pressure you run your tires at. I do know based on my own experiences and research that tire pressure affects a lot of things besides mileage.
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  #19  
Old 12-12-2013, 08:30 AM
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Re: Recommended Tire Pressures

I hope when people are saying over inflating, they are referring to the cars guide and NOT the tires max printed on the sidewall.
That would be suicidal to over inflate the tire max pressure.







Once you get a pressure too high, the tire starts to skip over road surfaces rather than complying to it. As a result it loses traction, no longer transferring full power to the road.
So a drop in grip, performance, tire wear, safety and milage.
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  #20  
Old 12-12-2013, 08:34 AM
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Re: Recommended Tire Pressures

Quote:
Originally Posted by bill_de View Post
It's the same book that dictates you should change your oil every 1,000 miles or 10,000 miles. And in both cases the answer is probably somewhere in the middle.

Higher air pressure will tend to wear tire centers. Low pressure will wear the edges. Tire companies have tested and proved this over time.

But, the exact 'proper' pressure is a little fuzzy. The tire has the 'maximum' pressure stamped on it. While this might be a conservative #, I would pay some attention to it. The auto maker gives a 'recommended' pressure. The auto maker needs to maximize mileage and handling while still keeping an eye on smooth ride.

When radials first became popular, many luxury manufactures were recommending 23 PSI. That gave radials a reputation for blowing out. But that Coupe de Ville was smooooth!

There is no perfect PSI, just don't ignore the guide lines.
What I do is to take my tire tread depth guage and check my tire every few months to see how they are wearing across the tread width. If I see the tread wearing more in the center then I lower the pressure. Over time I have found that the tire I have run works best at 35psi. Every tire is different so its really not a simple one size fits all conclusion. Not all tires wear the same so I have found my method to work best for me.
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  #21  
Old 12-12-2013, 10:23 AM
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Re: Recommended Tire Pressures

mswlogo, I could be wrong, but I think that 36 is also designated for the 20s...which I run at 38 cold normally as previously mentioned. With the low 20º F out there this morning, I suspect they are sitting there at about 33 psi, down from 35 psi yesterday afternoon when I looked, however... The air is shrinking to try and stay warm.
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  #22  
Old 12-12-2013, 10:35 AM
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Re: Recommended Tire Pressures

Why don't you just go all the way with Hypermiling and put 4 donut tires on your truck? Your rolling resistance will be very low.

In a few short months the people that are over-inflating their tires will start a thread that reads, "My car is starting to handle poorly" or "I lost control...."
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  #23  
Old 12-12-2013, 10:40 AM
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Re: Recommended Tire Pressures

I've been reading this thread, and just wondering, does anyone actually inflate their tires to the manufacturer's recommended psi? Is that a number they pull out of a hat, or is it actually something that has been tested and found to be good?
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  #24  
Old 12-12-2013, 10:42 AM
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Re: Recommended Tire Pressures

Typically for track days and drivers schools all seasons are overinflated to stiffen the sidewalls up. This does not effect handling. Typically tires are overinflated by 4 to 6psi.

Again chalk tires and see if you are rolling over on the sidewall.
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