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Old 01-21-2010, 10:50 AM   #1 (permalink)
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Default Tire Pressure Observations

Every month for work I have to drive about 1500 miles in a few days. It is LOTS of stop and go around town, with about about 700 miles of freeway. I dont try to get super high milage on these trips because I am in a big hurry, usually driving between 75 and 80 on the freeway.

Normally on these trips I run 45 PSI and get 50-52 MPG. This month because of the rain I ran 35 psi and only got 47-48 mpg. I just thought I would share some real world comparison of MPG vs tire pressure.
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Old 01-21-2010, 12:18 PM   #2 (permalink)
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Tyres filled with nitrogen rather than air are much more stable to tire pressure during a trip and loosing air pressure over time. They also keep the tires cooler than with natural air. The bikes hate it but Michelin is developing a fuel saver tire that is cool when driven straith and only get warmer/grippy in turns.
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Old 01-21-2010, 09:30 PM   #3 (permalink)
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Sorry to pop the Nitrogen bubble but,

Honda has addressed this recently in a Service News article.

_Except_ for _slightly_ lower permeability (smaller loss over time) the difference in a passenger car is zero. And except for a high quality fill (air purged out to less than 1%) which cannot be done by merely adding "pure" nitrogen at the valve stem, even this one "advantage" will be reduced. It does not eliminate the need for a semi annual 2-4 psi topping off of the tire pressure.

Now a in a NASCAR where the pit crew isn't merely wearing gloves for appearance (those tires fresh off a 200 MPH lap will burn you) a pure nitrogen fill makes the all the difference in the world. You'd either have to start off so underinflated to allow for the pressure increase as the tires heated that even a first lap at 100MPH would be difficult. Or risk (and expereince) many tire failures as a result of the overinflation that would result if starting "cold" with sufficient air pressure for a full speed run around the track. Nitrogen "solves" those issues.

Nitrogen better in a passenger car at speeds under 100 MPH? Only if its free. Else you've paid for more than you got.


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Old 01-22-2010, 07:18 PM   #4 (permalink)
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Quote:
Originally Posted by blown240 View Post
because of the rain I ran 35 psi
Uh, think about that for a minute. Why put more tire surface down and increase your chance of hydroplaning? More pressure reduces the risk of hydroplaning not less. What gets you on top of the water easier, a boogie board or a slalom ski?

Atmospheric air is already 78% nitrogen. Use in passenger tires is a waste of money.
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Old 01-23-2010, 08:06 AM   #5 (permalink)
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While more surface increases the risk of hydroplaning, it will improve grip. Most of the time in rain grip is the issue in teh Insight, not hydroplaning. Depends on road type/condition obviously.
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Old 01-23-2010, 08:29 AM   #6 (permalink)
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Quote:
Originally Posted by satchel View Post
Uh, think about that for a minute. Why put more tire surface down and increase your chance of hydroplaning? More pressure reduces the risk of hydroplaning not less. What gets you on top of the water easier, a boogie board or a slalom ski?

Atmospheric air is already 78% nitrogen. Use in passenger tires is a waste of money.
The insight tires are already very slim that hydroplaning is not an issue. In rain, less pressure resulting more grip is a good trade-off.

The problem with nitrogen fillings is that the air has to be sucked out first which my dealer didn't. But I could feel that the tire pressure was much more stable with cold tires compared to hot tires and the tire lost much slower air pressure over time which doens't mean you don't have to check it from time to time.

More stable air pressure means that you can put more pressure in cold tires to get the desired air pressure with warm tires. This should reduce the roll resistance. The critical question is how long do the tires take to get warm? On a highway, they soon get hot but around town? Maybe the relation of miles running with cold and those running with warm tires is very low. Therefore the whole effect could not be measured and it would be right to say it's a waste of money.
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Old 01-23-2010, 10:31 AM   #7 (permalink)
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If the OP is driving 80mph the last thing they want to do is lower their tire pressure. Sidewall flexing from low pressure and speed are what overheat tires. Higher pressure also enhances handling. At 35psi the tire is going to want to roll over on it's sidewall in cornering or emergency maneuvers and lose grip. You make my argument nayrhyno, if the tread doesn't reach the pavement through the water there is no grip. At 70-80mph hydroplaning does become an issue with any tire. Motorcycle hydroplanes over water#
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Old 01-23-2010, 11:03 AM   #8 (permalink)
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Quote:
Originally Posted by satchel View Post
If the OP is driving 80mph the last thing they want to do is lower their tire pressure. Sidewall flexing from low pressure and speed are what overheat tires. Higher pressure also enhances handling. At 35psi the tire is going to want to roll over on it's sidewall in cornering or emergency maneuvers and lose grip.
35 psi is not low pressure for auto passenger tires. What people have been using for pressures in the Insight for higher fuel economy reasons is considered high pressure. An Insight tire @ 50 psi has less rubber contact with the road compared to a tire @ 35 psi.

A picture is worth a thousand words....Check out the contact area of a properly inflated tire vs the other two options:



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Old 01-23-2010, 05:11 PM   #9 (permalink)
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That's not a picture, it's an exaggerated illustration. This is just an example of the propaganda the auto manufacturers and tire companies put out to get you to feel you haven't left your living room couch driving down the road and to sell more rubber by having you wear your tires out quicker.
Here is a realworld example, my left front tire with nearly 22K:


The sidewalls on my tires bulge more with less air than with more. Those illustrations are a bunch of hooey. That or they are from the days of bias-ply tires. The steel belts on modern tires don't allow the tire to bulge in the center of the tread. Judging by the track left on my garage floor when my tires are wet they lay down a full tread pattern.
My experience has been very good with 60#, sun, rain, snow and ice. I have more resistance to hydroplaning, better handling, cooler running at high speeds, more even and longer wear, and to top it off lower rolling resistance and thus better gas mileage.

And more realworld experience, (save the we're not driving fully loaded police car arguments, much of this applies to any car): Driving Under Pressure (full article) - CleanMPG Forums

Last edited by satchel; 01-23-2010 at 07:22 PM. Reason: correct mileage on tire, pic older than I thought
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Old 01-23-2010, 06:18 PM   #10 (permalink)
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Tyres need some flex to generate grip, it seems sensible to run lower when its raining and friction coefficients are better. At 40psi my insight feels like it has very little grip and Ive lost the back end a few times at low speed when its wet.

Tread depth will effect hydroplaning resistance far more than tyre pressure.
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