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Discussion Starter #1 (Edited)
Dealer or Factory put 28-29 PSI
Sticker on Driver Side door opening says 33 PSI
Tires say 44 PSI MAX

I just put 37 PSI in. Is that dangerous?
If not let's see if I improve from 41.3 MPG!
 

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Dealer or Factory put 28-29 PSI
Sticker on Driver Side door opening says 33 PSI
Tires say 44 PSI MAX

I just put 37 PSI in. Is that dangerous?
If not let's see if I improve from 41.3 MPG!
You will be fine :). I've had my I2 since May of 2009. I've gradually increased the tire pressure from factory to 36, 38, 40, and 41 PSI. I find that the ride and mileage is quite good at 39-40 PSI, so I keep them at that pressure.

The max sidewall is 44 PSI. This is the maximum pressure at which the factory recommends tires before bulging begins to occur. It is NOT the max pressure before explosion. Anyway, you need to take into account that the pressure inside the tire will increase due to the ideal gas law (pV = nRT). That is, the pressure times the volume is equal to two constants times the temperature. Since the volume remains the same but temperature increases due to road friction (heating up the air inside the tire), the pressure also increases to balance the equation.

That said, I'd imagine the pressure to increase a few PSI after sustained highway driving, but haven't done any calculations to quantify the amount. It won't be a huge amount though (just a few PSI), so inflating to about 40 PSI is fine.

In summary:
Modern tires are steel-belted tires. You are safe to run up to the max sidewall, but take off a few PSI to allow for increased pressure due to heat. Your ride will get more bumpy, but fuel economy will increase. In fact, it is better to run the pressure higher than the factory recommended in some cases, otherwise you will see increased wear on the outer edges of the tires due to sagging in the center.
 

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[snip]You are safe to run up to the max sidewall, but take off a few PSI to allow for increased pressure due to heat. Your ride will get more bumpy, but fuel economy will increase. In fact, it is better to run the pressure higher than the factory recommended in some cases, otherwise you will see increased wear on the outer edges of the tires due to sagging in the center.
You are safe to fill the tires to the max inflation pressure on the sidewall when COLD, the fact that they warm up while driving is factored in and there is even more of a fudge factor too, the tires are underrated to account for pretty much every condition including a few patches, some damage, and terrible roads. Many people in the 1st Gen forums run higher than the sidewall max and I haven't seen any issues come of it, but do so at your own accord. The fuel savings of running above what Honda suggests is significant, especially with the front of the car where it is heavier.
 

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You are safe to fill the tires to the max inflation pressure on the sidewall when COLD, the fact that they warm up while driving is factored in and there is even more of a fudge factor too, the tires are underrated to account for pretty much every condition including a few patches, some damage, and terrible roads. Many people in the 1st Gen forums run higher than the sidewall max and I haven't seen any issues come of it, but do so at your own accord. The fuel savings of running above what Honda suggests is significant, especially with the front of the car where it is heavier.
I wasn't aware - thank you for clearing this up!
 

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Going from 33-40psi, how many mpg do you typically gain? Is it significant?
 

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After 6 months of owning my Insight I've come to the conclusion that running higher than 36 psi does nothing for MPG. All it does is increase road noise and make a harsher ride. The best MPG gains can be made by adjusting the nut behind the wheel.(and running the ac as least as possible)
 

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Increased psi from 34 to 39

I have a brand new I2 ex Nav. I recently bumped the psi up to 39. I would say that the first tank after the bump had a small, but noticeable improvement in mileage. The road noise did increase, too.
 

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Are you suggesting that 42psi was not high enough? Cuz I'll put them up to 52psi if I need to to see some mileage improvement. I'll just have to be more carefull using the braks in inclement (rain) weather. Snow is an entirely different driving adventure.
 

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I have historically run higher than max psi pressure. I came from a Hybrid camry where I ran 55psi on all four corners. Traded in the Camry with the original tires after 86K miles, were not considered bald by any stretch of the imagination. I never had hydroplaning problems and the vehicle cornered much better. The few times I used the abs, it really handled itself marvelously.

Current psi levels on my new insight, 50psi front, 44psi rear. Ride is stiffer, but I have noticed our suspension sucks anyhow. The new dunlops are noisy, but I plan to counter that with some sound deadening. First tank averaged 48.2 with a mix of experimental driving, it has no where to go but up and we are using winter blend fuel.

I'm impressed already ...
 

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Are you suggesting that 42psi was not high enough? Cuz I'll put them up to 52psi if I need to to see some mileage improvement. I'll just have to be more carefull using the braks in inclement (rain) weather. Snow is an entirely different driving adventure.
I had my Insight now for 7 months and on suggestion of this forum I experimented with various tire pressures anywhere from the recommended factory pressure up to 46psi. All my driving has been done under 60 percent Highway and 40 percent city driving. At this point I can honestly say that there is no discernible difference in gas mileage. Gas mileage was actually very consistant for me the entire time only varying by 1 or 2 mpg which is probably from running the Air Conditioning more on Hot Days this summer.
I am getting right at 45-46 mpg regardless if my tires are at 32psi or 46 psi.
Don't pump up your tires to unsafe levels just to gain 1 or 2 miles more per gallon. It just isn't worth it or safe. And YES, overinflated tires do tend to hydroplane easier than properly inflated tires. If they are past the maximum inflation rate of the tire you also run the risk of having a blow out especially if you hit a pot hole. But I am sure others will disagree since there is always someone that knows better and some lemmings that will follow.
 

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The first statement you made about properly inflated tires will always be the point of contention ...

Running tires at placard levels may sound like the proper level, however, those levels tend to wear your tires sooner than later and decrease wet weather traction and performance compared to a tire that it inflated to sidewall max.

Side wall max inflated tires also last longer and do improve rolling resistance, however do not take my word, I will dig up some data related to a coast down test that was done with differing PSI's so you can see the difference yourself. You will be hard pressed to find articles directly related to tire failure and over inflation whereas under inflation is almost always a factor in failing tires.

PSI alone will not increase MPG's, how you drive when under those conditions will affect MPG's.
 

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Here was the data I was looking for;

The purpose of the test was to get an idea of the difference in coasting distances in my '98 Metro/Firefly over a range of tire pressures, 20-60 psi.

EDIT Jan 23/09: Posted a summary of this thread at: http://metrompg.com/posts/tire-press...resistance.htm

Tire info: 155/80/R13 Goodyear Invictas, rated 44 PSI max sidewall.

Placard info: vehicle placard recommends 32 PSI front/rear @ max load.

Pressure Gauge: readings were taken with an Accutire digital gauge, 5-99 PSI rated, with a manufacturer accuracy claim of +/- 1% + 0.5

Weather: 19C / 66F, 10 km/h SSW wind (roadway ran SW/NE)

Methodology:

* tires were pumped up to 60 PSI, drove to test route (< 5km), pressure adjusted
* car was driven up a small hill (approx. 6 ft. elevation, 8:1 slope), turned around and stopped at a marked point
* engine off, transmission in neutral, brakes were released
* car rolled down short hill onto a flat run-out road
* where the car stopped, the road was marked
* pressure was adjusted (dropped 5 PSI)
* rinse & repeat
* NOTE: only one run per PSI

I used a bicycle wheel to measure the rolling distances - counted revolutions from the starting point, then converted circumference to total feet.

Ideally, I would have simply measured the coast down distance from a constant speed at the same point on a level road, but my cruise control isn't working, and I didn't want to deal with the possibility of driver error (varying speeds). Even more ideally, I should have done multiple runs per pressure level.

Since the car accelerates from rest and coasts to a stop, the differences in pressure are amplified compared to a simple coast down test.

Raw results for PSI/feet traveled

20 / 479.3
25 / 524.8
30 / 621.0
35 / 621.0 Most people are here
40 / 639.6
45 / 687.5 Most tires max psi cold
50 / 702.0
55 / 699.3
60 / 702.0

From this data all that can be concluded is that a vehicle will coast longer in relation to increasing PSI. How you use this benefit affects you mpg's.
 

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Hydroplaning causes.

Read particularly the sections pertaining to underinflation (the typical cause of hydroplaning) and motorcycle tires. Higher inflation levels tend to lift the edges of the tire and allow a coarse approximation of the motorcycle tire contact patch.

Draw your own conclusions.

As for blowouts... those typically occur in the sidewall of a tire due to heat induced fatigue. The lower the pressure, the more the tire flexes in the sidewalls, and the higher the heat build up. If you've never done it before, try rapidly bending a metal paper clip in one place repeatedly. You'll get heat build up and the metal will snap there eventually. This is the same process. Higher pressures allow less sidewall flex.

Hitting a pothole may cause more of a suspension jar but you are less likely to damage the tire from pinching against the rim or sharp angle deforming a belt.
 

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To get a modern car / truck tire to lift its edges like a motorcycle tire due to over inflation, you would have to serious exceed max sidewall psi.

In addition using a motorcycle tire is a poor example because the tire is designed that way in order to obtain traction during hard cornering.
 
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