The science/physics/math/safety behind overinflated tires...

[Alphawolf]

Sometimes, things that seem obvious in our minds don't pan out when put to the test. An example would be driving with the tailgate down in a pick-up truck. Most every PU owner has heard that the tailgate acts as a large air brake and you will get better mileage by lowering it or removing it. That just sounds right when we think about it. But...The fact is, that driving with a tailgate down or missing actually hurts your MPG in a pick-up. This has been tested and demonstrated.

Now...About over inflating our tires on the Prius. It seems logical that it would save gas by over inflating our tires to reduce rolling resistance. But....Has anyone actually tested it? What kind of MPG increase might be expected by this change alone. And, it is known that over inflated tires wear out quicker in the middle of the footprint. So, if one drives say, 12,000 miles per year, how much money would be saved by over inflating vs. how much money would be lost by shorter tire life. Also, what about emergency maneuvering on an overinflated tire, and the propensity of an overinflated tire to hydroplane on water?

I'm just wondering if we are "robbing Peter to pay Paul" so to speak...

[wstander]

What most of us do is run the tires at or near maximum rated pressure, which is not the same as overinflation.

The OEM tires, as well as most replacements are rated at 44PSI max, so if I run mine at 44/42, I am still in the allowable zone.

[NuShrike]

It is not overinflated, and has been well demonstrated and documented over many years by many drivers. Why don't you try it yourself?

[Tadashi]

I run mine on 42 which will increase to 44 or 45 on a really hot day. I do not think this is over inflation, but I do notice a 5-10 mpg difference than if I run the recommended 35 psi.

[Alphawolf]

<div class='quotetop'>QUOTE(NuShrike @ Jun 27 2006, 07:25 PM) [snapback]277749[/snapback]</div>

It is not overinflated, and has been well demonstrated and documented over many years by many drivers. Why don't you try it yourself?
[/b]

Oh I am doing it! (42-40) But to exceed the Toyota recommended p.s.i. seems to me to be an "over-inflation" to some degree.

But, I just got thinking about it, and wondered, if, over say 30 or 40,000 miles of driving, if we will actually save any money due to quicker tire center tread wear out. And, it has to have some effect on emergency handling and hydroplaning on wet roads.

[tumbleweed]

I think no one would recommend "over inflated" tires but that would be over 44 psi. Most of the people who post here use around 40 or 42 pounds.

I really think a lot of tests have been done* and the results of some of them are available here and on other forums. Ok, here is what I can remember:

1. increasing the pressure increases the speed you can go before you hydroplane

2. The tires won't wear out in the middle if you inflate them to 42 psi, some people have noted they still tend to wear on the outsides first or wear evenly across the tires as they should

3. tire life may improve with more pressure as long as you don't exceed the maximum of 44 psi, if you go a bit over that when they get warm it should still be no problem

4. more pressure means a rougher ride and there will be a little less traction under most conditions, but I doubt you can tell the difference

5. increasing the pressure in my Honda Accord from 35 to 42 psi increased the mileage from 30 to 33 MPG on the highway, I think I performed a fairly objective test.

I don't know what the actual difference in mileage is on the Prius because I haven't done a good test of any kind but it does seem significant.


*Here for example is some good info on hydroplaning and tire pressure: http://www.nus.edu.sg/comcen/svu/pub...droplaning.pdf

[galaxee]

the inflation range is determined by the tire company, not toyota.

who knows more about tires- a company that makes them or a company that sells cars?

toyota's air pressure specs are pretty whacked in this case. as long as you're in the range according to the tire manufacturer, you're not overinflated.

[Prius Maximus]

qualifications:
My 2004 Prius is my 7th car. I have 75,000 miles on it. I am on my second set of tires on the Prius. I have driven over 400,000 miles in my life.

Standard operating procedures:
Pre-Prius: 32 PSI per car mfr recommendations. Prius: 44 psi all around.

observations:
Tire wear: at 32 psi, my tires were ALWAYS much more worn on the edges than the center (underinflation) and were bald by the rated treadwear mileage. At 44 psi, the OEM Badyears got 67,000 miles, could have gone at least another 5,000 and were worn evenly across the tread.

MPG:
The tire store installed my Hydroedges at 32 PSI (yes, I was too much in a hurry to check the pressure), after a 30 mile trip when I got home, my mpg for the trip was 41 mpg. I inflated to 44 PSI , and mileage went back up to like 48-51 (cold December chicago weather).

Safety:
75,000 miles at 44 psi, I have never skidded, lost control, had a blowout or flat. Even in several panic stops/emergency maneuvers.

Make up your own mind, but for me - 44 PSI saves gas, rubber, and is safe.

[jamarimutt]

<div class='quotetop'>QUOTE(galaxee @ Jun 27 2006, 09:41 PM) [snapback]277761[/snapback]</div>

the inflation range is determined by the tire company, not toyota.
[/b]

Read the tire's sidewall. I believe it says to use the manufacturer's recommended pressure.

Another point to consider is the effect of such high pressures on the harshness of the ride, suspension components and increased incidence of rattles.

[Prius Maximus]

<div class='quotetop'>QUOTE(jamarimutt @ Jun 27 2006, 08:54 PM) [snapback]277771[/snapback]</div>

Read the tire's sidewall. I believe it says to use the manufacturer's recommended pressure. [/b]

Nope. It says 44 psi max.

<div class='quotetop'>QUOTE(jamarimutt @ Jun 27 2006, 08:54 PM) [snapback]277771[/snapback]</div>

Another point to consider is the effect of such high pressures on the harshness of the ride, suspension components and increased incidence of rattles.[/b]

This is true. My ride is somewhat harsh, but I don't mind. My only real rattle is my sunglasses in the overhead if I don't put them in right.