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Old 10-26-2008, 03:40 PM   #1 (permalink)
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Tire pressure and fuel economy RESULTS

I searched through the many many tire threads for this and while I learned more about tires that I ever thought possible I didn't find anything quantifying results of different air pressures (underinflation, overinflation, and hyperinflation). MetroMPG, I read that you were planning to test and post results--did you ever do it?

A little context: I bought low RR tires for my truck and seemed to increase FE on average about 8%, but overinflating (even to just 35 psi) only seems to help the truck ride rougher than it already does. I haven't had the cash to buy a scanguage yet and am determining my FE old school (like the manual recommends): miles driven divided by gallons used. While a good system for an average FE, it isn't so good for showing the effects of small mods like this.

So I'm wondering how much tire pressures are helping anyone else because if the effect is negligable I'm going to put it back to help keep my coffee off my lap!


Last edited by 07b2300; 10-26-2008 at 03:59 PM..
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Old 10-26-2008, 04:04 PM   #2 (permalink)
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My tires are at max (50 psi) and I can tell when they are down a month or so later when I have to reinflate. My car coasts in neutral approx. 1/3 less with 35psi vs. 50 psi. There are times when I'm at max psi that I don't touch the gas pedal for close to a mile on my commute to work.

While I don't have stats to back this up the only two mods I made starting from 17 mpg before ecomodder was max psi and driving technique. When my tires are at 35psi my mpg is down 1-2%. I know it works...no doubt. I'm 33% over epa and I noticed 1-2 mpg gain after inflating my tires to 50 psi.

If you check my fuel log and look at the 27 mpg I pulled on a summer roadtrip I guarantee it is all due to P&G with 50spi. I used to get 22 mpg on those trips a year earlier.
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Old 10-26-2008, 05:00 PM   #3 (permalink)
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Quote:
Originally Posted by 07b2300 View Post
MetroMPG, I read that you were planning to test and post results--did you ever do it?
Is it what you are looking for ?
http://ecomodder.com/forum/showthrea...ures-2721.html

Quote:
A little context: I bought low RR tires for my truck and seemed to increase FE on average about 8%, but overinflating (even to just 35 psi) only seems to help the truck ride rougher than it already does.
From placard of 33 psi, I inflated to 43 and felt a change in coasting. When inflated to 46 I felt even more difference and in city it seamed to coast forever... I inflated to sidewall of 51 yesterday evening.

Quote:
I haven't had the cash to buy a scanguage yet and am determining my FE old school (like the manual recommends): miles driven divided by gallons used. While a good system for an average FE, it isn't so good for showing the effects of small mods like this.
http://ecomodder.com/forum/showthrea...auge-4369.html
Personally at each fill-up I calculate the saving in money, and I save that amount for investment in my cars. I already bought some anti-rain and anti-fog products, some new hubcaps, a voltmeter and a little solar panel (for the battery I had to change but that isn't totally dead, so I keep it to feed my (GPS) navigator).

Quote:
So I'm wondering how much tire pressures are helping anyone else because if the effect is negligable I'm going to put it back to help keep my coffee off my lap!
As numbers from MetroMPG post are significantly different, you can guess that with the proper driving techniques (DWB and coasting) you'll get success. Changing habits do need work, but success will be significant.

Denis.
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Old 10-27-2008, 01:42 AM   #4 (permalink)
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Quote:
Originally Posted by Matt Herring View Post
My tires are at max (50 psi) and I can tell when they are down a month or so later when I have to reinflate. My car coasts in neutral approx. 1/3 less with 35psi vs. 50 psi. There are times when I'm at max psi that I don't touch the gas pedal for close to a mile on my commute to work.

I don't have stats to back this up...
Heh! I'll back you up, mang...

And, you can take this from the World's Record Holder for Honda B16A2 FE!

50 psi is THE sweet spot for regular street tires... not 36 psi or even 44 psi... I've tried them all - door sticker to 50+ psi!

If you're a real tweak and run 'Land Speed Record Tires' - Bonny Tires - you can get away 60-70 psi, but I think that's getting a little extreme...

Anyway, pump them babies up to 50 psi and I guarantee you'll see that everything Matt Herring said above is 100% true!

Um...

I suppose it goes without saying - but we're talking auto tires here, not truck tires, which require considerably more psi...
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Old 10-27-2008, 02:44 AM   #5 (permalink)
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Quote:
Originally Posted by BlackDeuceCoupe View Post
50 psi is THE sweet spot for regular street tires... not 36 psi or even 44 psi... I've tried them all - door sticker to 50+ psi!
Is this with your tires rated to 50 psi? Mine are rated at 44, which is where I keep them. I know some are fine with overinflating, but I'm not as of yet.
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Old 10-27-2008, 02:55 AM   #6 (permalink)
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Quote:
Originally Posted by DoctorP82 View Post
Is this with your tires rated to 50 psi? Mine are rated at 44, which is where I keep them. I know some are fine with overinflating, but I'm not as of yet.
In MetroMPG's experiment, 44 psi rated tires are used and they get their better score at 50 psi.

Of course different brands, different results.

Denis.
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Old 10-27-2008, 08:39 AM   #7 (permalink)
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Quote:
Originally Posted by groar View Post
In MetroMPG's experiment, 44 psi rated tires are used and they get their better score at 50 psi.

Of course different brands, different results.

Denis.
That result could be well within the margin of error for the kind of testing that was done.

We know that rolling resistance as a function of inflation pressure varies more or less linearly. A tire with more inflation pressure will always have less, or at worst the same, rolling resistance the same tire with less inflation pressure.

How much it impacts FE depends on the vehicule, mainly what portion rolling resistance represents compared to other sources of drag, but you can safely assume that more pressure will result in the same of better fuel economy.
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Old 10-27-2008, 10:42 AM   #8 (permalink)
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This past Saturday morning I slugged down to the Toyota dealership to get the 'ole oil change. They nicely greet you at the door...process your car and then whisk you away to a comfy waiting area with 42" plasma TV's on the wall (full breakfast bar available if you want).

During the processing of my vehicle the "attendant" that does your paperwork (who is really a salesman) asked if I wanted the tires rotated. I obliged and mentioned that the tires are at 50spi and I'd appreciate if when they rotated them that they kept it at 50 psi (every time before they dropped it down to 35 psi without asking). When I mentioned the psi the attendant asked why I had them so high and here's the conversation (roughly what was said)...

Me: "at higher psi there is less rolling resistance which gives me better mpg's while I'm coasting in neutral."

Himafter a chuckle), "You believe in that stuff, huh?"

Me: "I've driven 1500 miles over epa for free since June partly because of my higher psi. Haven't you seen more and more SUV's available on your lot and less cars available since gas prices went up?"

Him: (in a robotic tone) "We can barely keep the SUV's on the lot and higher psi ruins your shocks and wears your tires out faster."

After this comment I expected the phone call an hour later to upsell new tires and shocks to me...thankfully it was only $25 for new wipers.

So with no interest from him in talking about saving gas I pedaled my feet away. And, as a fact (because I counted while I was waiting for my vehicle), they have exactly 3 used and 4 new prius cars available on the lot vs. 26 4runners, 42 tacomas and 28 tundras (and 22 FJ Cruisers). When I bought my 4runner in May of 2007 it was the only 4runner left on the lot!

Last edited by Matt Herring; 10-27-2008 at 10:44 AM.. Reason: Forgot to mention the 22 god-awful FJ Cruisers there too!
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Old 10-27-2008, 02:25 PM   #9 (permalink)
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Maybe its because i'm cheap, maybe its because i'm an engineer, maybe its because i'm honest... but i loath salesmen, and that's a funny story. I just hate that he says the same thing to everyone else and 9/10 believe him. I'm sure he'd tell you it was a lucky day that there were >100 guzzlers on the lot... total stroke of luck.
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Old 10-27-2008, 04:10 PM   #10 (permalink)
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Tyre pressure is directly related to the weight of the vehicle. 50 PSI in an SUV is safer than 50 PSI in a small hatch back. If you over inflate tyres you will have a smaller contact patch on the road hense the lower rolling resistance. This can create uneven wear, generally in the centre of the tyre, as this is the area with most contact with the road.

I do over inflate by a few PSI, but not much, because I don't want to compromise safety for a 1 or 2 extra mpg.

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