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Old 10-23-2011, 04:12 PM   #1
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Tire pressure difference

Just curious what the difference in tire pressure makes on mpg. Say, if you were running 10 psi below your max pressure, what percentage difference would that make in mpg? What if you were running 15 psi below max pressure? Or 20 psi below?

I'm just curioius what the difference really is, how significant a drop it causes. Any help?
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Old 10-23-2011, 05:16 PM   #2
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Re: Tire pressure difference

The effect of medium changes like that differ for every person/vehicle/tire/etc.

It's been so long since I have run less than the max that I can't even remember just how it affected my mileage.
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Old 10-23-2011, 05:47 PM   #3
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Re: Tire pressure difference

It is not going to be the same for every vehicle, as weight, tire size, sidewall size, stiffness of the tire, how you drive, the speeds you drive at and all that can cause variance, but, the EPA says, under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires.
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Old 10-24-2011, 05:49 AM   #4
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Re: Tire pressure difference

That's significant, .3 percent per psi underinflated.

It is my understanding that the lower it is, the percentage drop is exponentially greater the lower you get. At the same token, the higher you inflate gets into the law of diminishing returns. I've experienced this. There seems little difference between say 58 psi and 65-70 psi. If there is much difference, it's hard to quantify. However, the difference between 32 and 20 is huge as far as FE is concerned.
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Old 10-24-2011, 08:52 AM   #5
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Re: Tire pressure difference

Also I believe that lower than sidewall pressure affects the longevity of the tires.

Only times I would run lower than sidewall pressure is if I would be driving in a lot of rain or snow.
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Old 10-24-2011, 01:35 PM   #6
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Re: Tire pressure difference

Yes, lower sidewall pressure significantly affects the life of tires. More flexing in the sidewall builds heat and weakens them, there is more friction through road contact (more heat), and the air is able to expand easier, which means it is heated easier(more heat again for that poor sidewall).

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Originally Posted by benfrogg View Post
That's significant, .3 percent per psi underinflated.

It is my understanding that the lower it is, the percentage drop is exponentially greater the lower you get. At the same token, the higher you inflate gets into the law of diminishing returns. I've experienced this. There seems little difference between say 58 psi and 65-70 psi. If there is much difference, it's hard to quantify. However, the difference between 32 and 20 is huge as far as FE is concerned.
B
No, the losses would diminish as you get lower as well. The whole thing is on a curve really I would think. There is a point where your losses or gains would be highest, and generally speaking, MFR recommended tire pressure should be above that point where it is already generating diminishing returns on fuel economy gains, since they are balancing FE against road comfort and noise and against tire life, so they don't run tires at the maximum pressure. So, in the real world, I would think that FE losses would be increasing as you lose pressure to a point where your losses peak, and then you start losing fuel economy at a lower rate after that.

Actually, that sounds like it would be fun to write a paper on. I should thank you guys for having this discussion..... Thank you.
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Old 10-24-2011, 09:48 PM   #7
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Re: Tire pressure difference

Quote:
Originally Posted by FIND View Post
It is not going to be the same for every vehicle, as weight, tire size, sidewall size, stiffness of the tire, how you drive, the speeds you drive at and all that can cause variance, but, the EPA says, under-inflated tires can lower gas mileage by 0.3 percent for every 1 psi drop in pressure of all four tires.
This is really helpful information. You're like an encyclopedia, and I appreciate the help!
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Old 10-25-2011, 07:46 AM   #8
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Re: Tire pressure difference

I looked that up....
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Old 10-25-2011, 10:37 AM   #9
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Re: Tire pressure difference

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Originally Posted by FIND View Post
I looked that up....
So you are a wikipedia.
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Old 10-25-2011, 02:50 PM   #10
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Re: Tire pressure difference

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So you are a wikipedia.
No, cause 14 year olds don't edit me in the middle of the night as a prank.
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