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

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...

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.

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

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.

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

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.

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: www.nus.edu.sg/comcen/svu/publications/hpc_nus/dec_2005/hydroplaning.pdf

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.

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.

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

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.

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

Nope. It says 44 psi max.

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

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.

Here is something I came across concerning this thread.

Any comments, thoughts?

Autozone Reference Library - Tires

I have been reading Prius posts here and elsewhere for a very long time, but never yet one that spoke of center tread wearing more rapidly than the edges. There have been plenty of 'edge wear' posts, of sourse.

The belt construction of radial tires make it very difficult for the center of the tread to bulge out. Perhaps one could achieve this, but not at any pressures at or below what is says on the tire sidewall.

In the Prius Milage Record run of >100 mpg the "well worn" tires were at 60 psi. Please check me if I am wrong but that is what I remember. " In the absence of Data you can have any opinion you wish". That was outside normal driving but it is documented.

<div class='quotetop'>QUOTE(Alphawolf @ Jun 27 2006, 08:51 PM) [snapback]277729[/snapback]</div>

Of course there are tradeoffs and we give up somethings to gain others.

From higher pressures we _generally_ give up some:

• ride comfort
• braking traction on dry roads
• acceleration traction on dry roads
• less rattles developing
• safety from blowouts from hitting potholes or other road hazards?
• less wear on the suspension

From higher pressures we generally gain:

• slightly lower fuel consumption and emissions
• less hydroplaning
• better traction in shallow snow and slush
• more responsive steering
• safety from blowouts from tires overheating at high speeds and/or under heavy loads
• safety from rim damage hitting potholes

I like to think of each of these objectives as concave (down) functions of tire pressure, each having its own unique optimal tire pressure. Clearly riding on flat tires (zero pressure) or tire tread glued to 24" flat rims (or tires magically inflated to infinite pressure without blowouts) wouldn't serve any purpose.

Finding the tire pressure that is optimal overall depends on what weight we give to the different objectives or criteria. As our preferences vary from person to person, so will the optimal tire pressure.

As for me, I'm currently running Continental ContiProContact tires at 50/47 psi recently. (The maximum tire pressure for these is 51 psi.) These are very soft grand touring tires with grippy tread but soft sidewalls and the ride is still comfortable. I'm keeping them inflated this high partly to reduce hydroplaning, as we've been having plenty of rain (and even more forecasts of rain) in Boston. I plan to lower them when the rain goes away (in a week or two God willing) and look forward to seeing the effect on fuel consumption, ride, handling, noise, traction, etc.

<div class='quotetop'>QUOTE(theorist @ Jun 28 2006, 12:27 PM) [snapback]278080[/snapback]</div>

Just a couple of notes to add to this thread about overinflation.

1) Toyota specifies the recommended minimum tire pressure I believe, - not maximum is that right? In either case, it is set to compromise ride quality with tire wear and we all know it is whacked up because all of our tires wear on the outside (either suspension characteristics or underinflation).

2) All tire manufacturers state the maximum inflation pressure as COLD TEMP inflation. So - at rest, cold, your tires can be set safely to 44psi if that is the max pressure on the sidewall. A rise in pressure with heat and road friction is expected in those numbers. That's why you don't fill the tires when warm if you can avoid it. There is no overinflation going on here.

3) yes there is some road feel fed back into the harder tires, but I found this totally goes away with the Comfortreds. The Integrities are a cheap tire and ride like one. Those who have upgraded have seen dramatic improvements in wet and dry operation, as well as comfort.

There is no question in my mind that the tires have to run harder to get thebest mileage. I run mine at 42/40 and have had literally no problems. My Integrities ran 35K miles that way and had edge wear, not center wear. Remember that the wear relates to the car weight on the wheel as well. The heavier the car, the more a high pressure tire will wear along the center line - the Prius will never push a good tire that far.

Some drivers might - but the car won't!

Gene

i have run 42/40 on my car and my last oil change my tires were measured at 7 mm inside, outside and center on all 4 tires. i have my oil changed at Toyota of Olympia. the first time i took it there, they changed tire pressures to 35 psi. the 2nd time i took it there, i requested they not change the tire pressure, they have not touched it since.

Tires pressure ratings are always done "cold". Tire pressure rating are determined by the tire manufacturer.

What are people's thoughts of driving with maximum (e.g. 42 psi) inflated tires in the snow?

I would think that during the winter months, one would want to lower tire pressures to 'Toyota recommended' ranges for better traction in ice and snow. Safety over mpg?

It's much more dangerous to have underinflated tires. It's difficult to overinflate your tires to an unsafe level, if you follow the max PSI on the tire sidewall.

The tire manufacturer lists the cold pressure, they allow for thermal expansion and a big safety margin, so running at max PSI still has a big safety margin.

Some people inflate beyond the sidewall max pressure. I don't think that's the best idea, but it works for them.

Car makers list lower pressures for a better ride and other factors, they are concerned about the total performance of the car, and not just that of the tires (which do better at higher pressure)

<div class='quotetop'>QUOTE(Alphawolf @ Jun 27 2006, 05:51 PM) [snapback]277729[/snapback]</div>

Well, as long as we're reconsidering "things that seem obvious in our minds", let's challenge this one.

Yes, I, too, have seen the pictures in tire shops of various kinds of "irregular" tire wear: bald on the edges, bald in the middle, scalloped, etc. But I've had 37 cars, have inflated the tires above mfr. recommendations on all of them (even pre-radial, bias-ply tires) and have NEVER seen extra wear in the middle on a tire.

On my '02 Prius, I ran the OEM Bridgestone Potenzas (by general acclamation, even worse than the Integrities) at 42/40 for 53,000 mi. and when I replaced them they easily had another 5K in them. I had rotated them every 5K as per the manual. The tread wear was even from shoulder to shoulder.