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Old 04-04-2011, 09:40 PM   #1
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Tire Over-Inflation

I was reading some profiles tonight and saw that many drivers have over-inflated their tires from 45 psi to 55 psi. I'm wondering why this is done? Does this improve a car's fuel economy and increase mpg? Does this not shorten the lifespan of the tire?

Please excuse my greenhorn questions. I'm new to this website and I'm just starting to look for a very efficient mpg car again. I had a 1980 Honda Civic when I was in high school and it got 38 mpg in the city and 50 mpg on the highway. I miss that car, and I've decided I want to drive something that gets similar mpg again. Anyway, I would love to hear the answers to my questions, and any other tips/advice anyone might have to extend my miles per gallon with my current vehicle.

Thanks to all!
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Old 04-05-2011, 02:07 AM   #2
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Re: Tire Over-Inflation

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Originally Posted by Arky View Post
I was reading some profiles tonight and saw that many drivers have over-inflated their tires from 45 psi to 55 psi. I'm wondering why this is done? Does this improve a car's fuel economy and increase mpg? Does this not shorten the lifespan of the tire?

Please excuse my greenhorn questions. I'm new to this website and I'm just starting to look for a very efficient mpg car again. I had a 1980 Honda Civic when I was in high school and it got 38 mpg in the city and 50 mpg on the highway. I miss that car, and I've decided I want to drive something that gets similar mpg again. Anyway, I would love to hear the answers to my questions, and any other tips/advice anyone might have to extend my miles per gallon with my current vehicle.

Thanks to all!
There has been a long and sometimes acrimonious debate about inflation pressure in tires.

While it is true that using higher inflation pressure in tires reduces rolling resistance (and therefore improves fuel economy), some point out that this may also lead to ride and traction problems, especially in the wet. The long term durability and wear aspects are still under debate. I suspect this is because there are a lot of factors involved - and in the case of durability, you need a huge sample size to get meaningful data.

But there are other things that can be done with tires that may have more benefits without the drawbacks inflation pressure has.

Careful selection of tires: Rolling Resistance, Treadwear, and Traction are tradeoff in tires. If you want to maximize one of those 3 properties, then you have to sacrifice one of the others (or both). The key is to find the compromise that works for you.

Using larger tires: The effect is small, but larger tires have better RR all other things being equal. So going from a 175/70R14 to a 185/70R14 is directionally better. But be careful, the effect is so small that it is easily overwhelmed by tire selection - PLUS, you need to be careful about changing rim diameter as the increased mass of the rim might have detrimental effects (but there isn't any data to quantify the effect).
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Old 04-05-2011, 04:42 AM   #3
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Re: Tire Over-Inflation

My feeling is this, if you want to run higher pressures, purchase a tire that is designed to operate within the desired pressure range you want to operate in. Personally, I put E rated tires on my 1/2 ton pickup truck and run them @ 70 PSI (max sidewall pressure 80 PSI). In any event, no matter what tire you choose, select a pressure that allows the tire to wear flat & even. This gets you the most life out of your tire, and the best traction for accelerating, stops, corners, braking, etc. If you over inflate to the point that you're only using a small strip of rubber in the center of the tire, it will wear out much more quickly. It may save fuel, but not enough to cover the cost of replacing tires that prematurely wear out because of over inflation.
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Old 04-05-2011, 04:53 AM   #4
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Re: Tire Over-Inflation

I ran 50psi for almost 3.5 years on a set of Kumho Solus touring tires on my '96 Saturn. Max sidewall I think was 42psi... They still looked new after the 27k miles and 3.5 years I put on them, and whatever wear had occured was even and smooth.

I feel strongly that running those tires that high made a big improvement on my overall gas mileage. My current SL1 has some type of Douglas All-Season tires that I'm running at 45psi.
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Old 04-05-2011, 05:47 AM   #5
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Re: Tire Over-Inflation

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Originally Posted by CapriRacer View Post
Using larger tires: The effect is small, but larger tires have better RR all other things being equal. So going from a 175/70R14 to a 185/70R14 is directionally better. But be careful, the effect is so small that it is easily overwhelmed by tire selection -
You may be surprised to learn why a larger tire will have lower rolling resistance: Because, at the same pressure as the previous size, it is overinflated. If you look up load-inflation charts you'll see that increased size brings increased load capacity or decreased pressure for the same load.

However, beware changes in gearing. A taller tire reduces your RPM at any given speed. In most modern manual transmission cars this is an advantage. In other cars it may be a disadvantage.

It also changes the height of the car, affecting aerodynamic drag.

Quote:
PLUS, you need to be careful about changing rim diameter as the increased mass of the rim might have detrimental effects (but there isn't any data to quantify the effect).
Increasing your rim size at the same time reverses the aforementioned effect on RR. Your increase in tire diameter giveth while your increase in rim diameter taketh away.

Although not an exact way to look at it, if you consider the mass of air in the tire rather than the size of the tire, you will see consistent results. By increasing the mass of air, either by increasing pressure or tire height, you will reduce RR (increasing width may work too but it may bring additional variables to the equation; that said, I'm a fan of increasing width).
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Old 04-05-2011, 06:09 AM   #6
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Re: Tire Over-Inflation

I have more info, including links to back up my claims, in the tire pressure link in my sig.

I began using increased inflation pressure long before I cared about fuel usage, when I was just concerned about replacing tires. My driving was wearing the outer edges, my disregard for road hazards was blowing out sidewalls, and the long-treadwear tires I bought started to have cracking sidewalls within a couple years of being new just from having so many highway miles on them. Then, after replacing them, I realized I could try increased pressure.

Wow! What a difference it made! My sloppy handling crisped right up. No longer did my tires squeal around every turn and the car plow straight when I wanted to turn. Instead they just stuck right to the road. I never had another blowout either.

80,000 miles later the tread was wearing evenly and I was never going back to using the manufacturer's recommended pressure. Then the Ford/Firestone fiasco happened and I decided not to even TRUST the manufacturer's recommended pressure, which seems to be made by the marketing department rather than the engineers.

Here's some of the common concerns that people have about increased inflation:

1. Handling traction: Improved. This is why autocross racers often inflate their regular street tires to 60psi for the race.

2. Uneven wear: In extreme cases you get some center wear. So what? Unless it's so severe that you've got 7/32 on the outside and bald in the center, you'll be able to safely continue using the tires as the worn center will still displace water to the tread at the sides, which will then channel the water away.

3. Tread wear lifespan: Increased pressure has ALWAYS increased total tread life for me. By the time I get visible center wear, I would have completely bald tires at the lower pressure. I guess YMMV.

4. Blowouts: These are not thin bicycle tires with tubes and loosely hooked beads. The most common blowouts are from insufficient pressure, and from snakebite punctures when the rim pinches the tire sidewall at a pothole/curb. Both blowouts are prevented by increased pressure.

My recommendation: Experiment to find the best pressure between manufacturer's recommended pressure and tire sidewall's maximum. For me it's always the maximum. If handling is too crisp then back it off. If ride is too rough then back it off. Check your wear often; if you find too much center wear then back it off a little.

I promise you won't crash and burn just from inflating to 45 instead of 35.
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Old 04-05-2011, 06:56 AM   #7
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Re: Tire Over-Inflation

well, here is my 2 cents

I recently replaced my tires on my car. I think they have been on the car for about 5 years (3 of those, I was on gassavers). I have logged in quite a few miles in those 5 years. I would say close to 75,000 or so. I replaced them because they were dry rotting. they were worn don't get me wrong but they still had quite a bit of tread left on them. they would have been fine for another year or two if they weren't all cracked. they were not leaking air but to be on the safe side, I replaced them

I run 45psi and my sidewall max is 51. the recommended is 32 (like most cars). I have never had a problem with tires wearing in the middle. I will say that I drive slow and use my brakes sparingly as it wastes momentum. if you are driving with fuel economy in mind, you usually aren't using the handling capabilities of the tires anyway.

regardless of all that, good luck on your quest for better MPG.
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Old 04-05-2011, 10:49 PM   #8
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Re: Tire Over-Inflation

Great answers!!! Thank you for all the answers, advice and suggestions. I'm going to put them to use and see how it works out for me. I suspect I will soon get a little boost in MPG because of this one small detail that I didn't realize before. Thanks again!
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Old 04-06-2011, 03:38 AM   #9
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Re: Tire Over-Inflation

I ran my VX tires at max sidewall pressure (4 psi) for 2 years. The car seemed to understeer more, and the ride suffered. Still, the tires showed almost zero wear after 40,000 miles (80,000 mile tire warranty). I was also getting mpg in the mid 50s to mid 60s on most days, with a high of 77 mpg. For me, it was worth it. And I didn't have to buy LRR tires.
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Old 04-06-2011, 04:55 AM   #10
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Re: Tire Over-Inflation

Sticking to the tire pressure recommended on the inside of the drivers door is probably why I seem to only get half the tire warranty worth before I have to replace.

I have since air the tires to close to maximum wall pressure.
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