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Old 10-06-2014, 09:13 AM   #1 (permalink)
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What is the optimum tyre pressure for fuel economy?

Hello everyone,

I used to own a car with 17 inch wheels and in the owners manuel it said that 38 PSI was the best pressure for economy.

I now own a car with 15 inch wheels and I have been over inflating all 4 tyres to 38 psi.

I 38 psi optimum for fuel economy? making the tyres have more rolling resistance with a higher psi?

What is the optimum for a 15 inch tyre?

I saw a myth busters episode and they said 30 psi was best.

Or does it depend on the wheel and tyre size?

Any help is much appreciated thank you

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Old 10-06-2014, 09:59 AM   #2 (permalink)
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30 is definitely not best for minimum rolling resistance... I think you might be mis-remembering that episode.

FYI:



Source: http://ecomodder.com/forum/showthrea...tml#post265379



Source: http://ecomodder.com/forum/showthrea...tire-2721.html
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Old 10-06-2014, 10:59 AM   #3 (permalink)
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Thanks for replying!

So 55 psi would be optimum?

I know my spare wheel in my boot the space saver is about 60 psi.

I would be concerned about the tyres at that pressure. Could they blow out at speed? Would I have to check with the manufacturer?

How many people on here are rocking tyres over 50 psi? Any problems?

Thank you
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Old 10-06-2014, 11:15 AM   #4 (permalink)
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The test shows higher pressure is better period. How high you want to go is up to you. I typically just pump mine up to 50 psi and call it good enough. There is diminishing returns the higher you go.
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Old 10-06-2014, 11:56 AM   #5 (permalink)
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I have mine set to 5 over sidewall max in the summer and 10 over that in the winter.
One of the best mods you can do.
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Old 10-06-2014, 12:26 PM   #6 (permalink)
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Quote:
Originally Posted by mada88 View Post
Thanks for replying!

So 55 psi would be optimum?

I know my spare wheel in my boot the space saver is about 60 psi.

I would be concerned about the tyres at that pressure. Could they blow out at speed? Would I have to check with the manufacturer?

How many people on here are rocking tyres over 50 psi? Any problems?

Thank you
Maybe spend some time using the search button.....but use 'tire'

probably over 100 threads on increasing tire psi for better mileage.
it would save having all those w/ high psi having to answer here to relive your concerns.
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Old 10-06-2014, 07:03 PM   #7 (permalink)
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Quote:
Originally Posted by dirtydave View Post
I have mine set to 5 over sidewall max in the summer and 10 over that in the winter.
One of the best mods you can do.
Are you sure you didn't swap the numbers around by mistake? I thought normally a person would put the pressure lower in the winter than in the summer, for traction.
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Old 10-06-2014, 08:52 PM   #8 (permalink)
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Quote:
Originally Posted by 101Volts View Post
Are you sure you didn't swap the numbers around by mistake? I thought normally a person would put the pressure lower in the winter than in the summer, for traction.
Nope. Google search winter rally tires.

Also running 35 psi on a 150 degree asphalt raises tire pressure from heat expanding the oxygen. When winter hits in Texas without touching the tires I lose about 10psi from the abient temp. The tire also does not get up to the same heat from friction of the road, that is to say the rolling resistance generates heat as well as getting heated from contacting the 150 degree pavement. That 35 psi in the morning can turn into 50psi really quickly on a long drive. However 35psi on a 35 degree pavement with a 35 ambient temp really won't affect the tire pressure so you would need to add more air pressure to get that same pressure youre running on long drives in the summer

Running nitrogen in the tires would cure both problems and tire pressure would remain constant through the temperature changes as it is not as heat sensitive as oxygen molecules

So yes I'm sure he's correct as I am forced to check pressure as Texas really only has summer and two months of winter every year lol
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Old 10-06-2014, 08:52 PM   #9 (permalink)
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Quote:
Originally Posted by 101Volts View Post
Are you sure you didn't swap the numbers around by mistake? I thought normally a person would put the pressure lower in the winter than in the summer, for traction.
Depending on the type of snow/ice cover on the road a higher psi and smaller footprint could give better traction due to higher ground contact pressure. Same resaon a skinnier tire works better than a wide one in snow (unless you are talking bottomless snow offroad, where those awesome polar exploration trucks use gargantuan tires for flotation.
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Old 10-07-2014, 07:24 AM   #10 (permalink)
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Quote:
Originally Posted by mada88 View Post
Hello everyone,

I used to own a car with 17 inch wheels and in the owners manuel it said that 38 PSI was the best pressure for economy.

I now own a car with 15 inch wheels and I have been over inflating all 4 tyres to 38 psi.

I 38 psi optimum for fuel economy? making the tyres have more rolling resistance with a higher psi?

What is the optimum for a 15 inch tyre?

I saw a myth busters episode and they said 30 psi was best.

Or does it depend on the wheel and tyre size?

Any help is much appreciated thank you
First, in the US, every car has a vehicle tire placard which lists the original tire size and the specified pressure for that size. I believe other countries use a placard as well - may be even the same one. Since 2008 the placard is supposed to be on the driver's doorframe, but prior to 2008 it could also be found in the glove box, on the fuel filler door or in the trunk (boot).

Second, "optimum" means there is a peak value - and for RR, increased pressure always results in better RR - up until the tire explodes. However, beyond a certain point, the increases in pressure don't result in much improvement in RR.

Third, the maximum pressure listed on the sidewall of a tire is exactly that - a maximum. It is NOT a recommendation. It also is relatively arbitrary - which I explain here:

Barry's Tire Tech - Load Tables

About 1/3 of the way down, I talk about "The notes on page 1-34", which deal will the maximum pressure. Others may disagree, but I think the maximum pressure is not something that tire manufacturers do a lot of thinking about as the load table is what is important and the load tabl;e defines the relationship between pressure and load.

Fourth, while RR improves with inflation pressure, other properties don't. The most obvious is traction which likely peaks, then goes down. This is the result of the shrinking contact patch. The problem here is that this is not well documented. There aren't charts for the literally thousands of different tires out there showing where that peak is. What we have is anecdotes.

Also, tire wear is somewhat tied to inflation pressure, with increasing inflation pressure causing more wear in the center of the tread. Again, this is not well documented - in fact hardly documented at all. All we seem to have is those ubiquitous anecdotes.

Other properties that seem to be adversely affected by increased inflation pressure are groove wander (aka tramlining), puncture resistant, impact resistant, and durability - again, anecdotal.

And lastly, I am NOT a fan of using a lot of inflation pressure, mainly because of the compromise in traction. So I don't recommend using more than 5 psi over the placard pressure. Other people have different opinions.

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