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Discussion Starter #1
Folks, heads up!

My pressure used to be 42/40. Then I had an oil change. Couldn't figure out how my mileage had dropped so badly (46/47ish).

Checked my tires. Not a single one had even 30 PSI. In the front, they were 29.5 PSI, and the rears were 29.0PSI.

I think my dealer checks the air when they do the tire rotation, and they probably let air out. So word to the wise: check your air pressure, especially after an oil change/tire rotation!
 

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It's also gotten colder in many places. Probably much colder than the last time most people checked the air in their tires...

Anyhow, a tire looses 1psi of pressure for every 10^F drop in outside temperature.

I think it was in the 70s when I last checked my tire pressure... Now the temperatures are in the 30s here. So, I expect my tire pressure to have dropped about 4psi on each tire...

If the snow clears up, I'll be raising the pressures on my tires this weekend...
 

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I see some pretty scary posts on this forum over tyre pressures. I am a car engineer and I would NEVER raise tyre pressures just to improve fuel consumption. It is dangerous. The loss of grip will be substantial, which can make all the difference between having a crash and avoiding one. You should stick to the manufacturers recommendations, they've been designing them longer than we have been driving them.
Over here in the UK it is illegal to run tyre pressures away from manufacturers recommendations and whilst this is a bit of an imposition it is as sensible as wearing a crash helmet on a motorcycle!
 

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Tire Pressure

I had similar experiences after dealer service. They keep letting the air out.

About a week ago, I got the Prius back; and the service writer told me that the tires were pumped up to 44/40, just like I specified. (I had told him about every other time, getting the car back with about 36 psi all around.)

After a couple of days, the mpg had dropped from about 51 to 46; so I checked the tire pressure. Sure enough -- 36 psi. I pumped them back up to 44/40 and am now back up to around 51 around town, again.

By the way, here in AZ, more tires fail from under-inflation than from blowouts from high pressures. Under inflated tires are dangerous in the summer heat. Don't you remember the Ford Exploders with Firestones? We had several deaths in AZ (with all types of SUVs), until the media started telling everyone to pump their tires up.
 

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Discussion Starter #5
I believe my sidewalls say 45 PSI max cold. The stem caps say 32 PSI. This is ridiculously low anyhow, but to make matters worse, when I got the car back from the oil change, it didn't even have 30 PSI in any of them!

As a practice, I pump the fronts up to 41 and the rears up to 40. If they happen to go a bit higher, I don't sweat it; I leave it alone. I drive mostly around town these days, with occasional freeway jaunts. It is my understanding too, that low pressure is more dangerous than high pressure. As for tire grip, I'm not too worried. We get lots of heavy rainstorms in Florida. I've never had the feeling like I've lost grip at the higher pressures.

And it's the difference between 45-48 MPG with 'em low and 53-55mpg with 'em higher.

Once again, I am not exceeding the sidewall figure.
 

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Overinflation and underinflation are both dangerous. The Explorer story is about cost cutting, the tyres were too marginal for the weight. Low pressure leads to risks of overheating and failure, but with a normal tyre and car this is when pressures drop below 20psi. Typically road car tyres are in their best grip area between 25 and 35psi. The best for your car will have been found under test by the manufacturer and be in the handbook. Normally adding 3psi or so is recommended when fully laden, for obvious reasons. I own 2 cars at present and both use pressures in the low 30s. Tyres at high pressure lose grip and have poor braking.
FYI road racing cars use very low pressures typically 18psi front 16psi rear. The only high pressure tyres used on race cars are the outer tyres used on Super Speedways, ie oval racing at 200mph, these are around 40psi. The inboard tyres run lower.
I do not dispute that higher pressures improve fuel consumption, I am simply trying to alert non engineering car owners to the very real risk of running tyres at a pressure too high for the vehicle, risk of skidding and poor wheel lock under braking. You may never be aware of it before you are in a hazardous situation, then you will be in as bad a situation as a SUV driver!
The maximum pressure moulded onto the tyre sidewall is the maximum safe pressure for the tyre design, this will probably only be used if the tyre is fitted to a very heavy vehicle. The Prius is not heavy, Toyota are very sensitive to the economy of this vehicle and will have recommended the highest pressure they could before losing grip.
 

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Note: usually when you have your
tires rotated at the dealer, you
probably drove on them for a while
to get to the dealer. So, right
there the measured pressure is not
at "cold."

If the weather outside is cold, and
the dealer measures the air pressure
when the car has been in their nice
heated garage for a while for servicing,
the air pressure will read higher
than it will once it gets outside
into the cold.

So, it is possible for the dealer
to put in 44psi at the shop, but
since it wasn't a true cold temperature
reading, when it gets measured
later by you it could easily read
many psi lower.

I had this happen in the worst case
last winter. Dealer refuses to put
anything higher than 32psi into
tires during the winter, claiming
they don't want to be responsible
for a blowout when driven over a
sharp pothole. Now, I had just
driven the car ~9miles to get there
for the service (warm tires), and
their garage was in the high 60s^F
while it was at the warmest in the
20s^F outside... Ignoring that the
tires weren't measured at cold, and
just going by the garage/outside
temperature differential, (60-20)/10=4,
if they set the tires at 32 when the
tires got outside they'd be at 28psi.
Definately no where near the 35/33psi
that Toyota recommends! It was like
driving on flats for the next day
or so until I got a chance to up
the pressures back to my usual of
around 40/38psi.
 

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Mr. f1eng,
Please describe your basis for claiming that the Prius is not a heavy vehicle for the size tire it uses. Many others have claimed just the opposite. In fact, at the Toyota recommended pressure, a large number of owners have had trouble with excessive edge wear, a sure sign of underinflation. Please provide all relevant measurements for your claims of danger at higher than Toyota recommended inflation, specifically for the Prius, not some generalization from other cars which have larger tires relative to their weight. Thanks.

I ask this because you claim to be a car engineer (what specialty?), and yet your advice goes counter to large numbers of other posters and my own observations. I will reserve judgement awaiting your evidence.
 

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Discussion Starter #9
Thank you, RSnyder.

Looking at it from a slightly different angle than you are, I also have been thinking something's not quite right here. The Prius is approximately 3,800 pounds. With me in it, let's just call it two tons. Assuming that that IS a "light" weight for those tires, I cannot see how higher pressures would pose a higher risk of blowout. If the car were significantly heavy, I might be convinced, but no, not for two-tons.

At higher pressures, it stands to reason that there is a more firm, more full "footprint" (contact patch) because the center tread is pressed outward at higher pressures. But even if it's pressed outward, nobody can convince me that inflating to the sidewall max might cause my car to "rise up" off of the edges. Sorrry, it's just not gonna happen!

If I inflated to something grossly over sidewall max, then I am operating the tire beyond its design specifications, and all bets are off. But I'm not doing this!

RSnyder, in conjunction with your observations about tire life, in my experience, on any and every car I've ever owned, using the auto manufacturer's PSI recommendation has always resulted in premature edge wear. Inflation to (or near to) sidewall max has always evened out that wear for me on my cars, and allowed my tires to last longer.

Maybe this is because I mostly drive my cars with just me in them, with little or no additional weight outside of a change of clothing or a laptop computer.

I will continue to run with higher pressures. I believe gas mileage is significantly better, and I believe my contact patch is better, providing for better control during maneuvering and braking.
 

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I am a race car engineer.
The Prius weighs 1.3 tonnes.
Uneven wear is not an indication of underinflation, for best grip there will be contact patch movement and uneven wear I'm afraid.
I am just giving a "heads up". Your Prius has less grip at higher than recommended pressures despite better fuel consumption.
Don't believe me if you like. I have 30 successful years with racing cars (several World Championships) nobody I have read on the list knows anything about vehicle dynamics.
I recommend using Toyota's recommended tyre pressures, thats all. This is based on sound data.
It is your life!
Sorry I mentioned it.
 

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f1eng said:
I am a race car engineer.
The Prius weighs 1.3 tonnes.
Uneven wear is not an indication of underinflation, for best grip there will be contact patch movement and uneven wear I'm afraid.
I am just giving a "heads up". Your Prius has less grip at higher than recommended pressures despite better fuel consumption.
Don't believe me if you like. I have 30 successful years with racing cars (several World Championships) nobody I have read on the list knows anything about vehicle dynamics.
I recommend using Toyota's recommended tyre pressures, thats all. This is based on sound data.
It is your life!
Sorry I mentioned it.
Ok, after all that I have to ask... Is it really spelled Tyre in the UK?
 

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Yes tyre is the English tire the USian spelling. I travel a lot and have been in the US a great deal, Indy cars, CART and Sports Car racing. I made a spread sheet years ago that I have now lost, it lists the differences between US English and actual English and caused great amusement amongst my friends here and there. Sadly now lost but you would be amazed by some of the differences!
 

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f1eng,
Over 90% of my driving is on straight and gently curved interstates, hardly the conditions that would cause a lot of contact patch movement. With the original tires I experienced a lot of edge wear at 35/33. I went up to 42/40 and wear was much more even. Later I replaced those tires with wider tires able to handle more load. They have a max inflation of 35, so I keep them at 33/31. They only experience slight edge wear. They do lose grip at 38 PSI (I tried that as a brief experiment). The original, narrower tires did not lose grip even at 42/40 (at least not noticeably).

Anyway, I appreciate the heads up, but it still seems like you're not taking the Prius tire size vs. weight into account.
 

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Robert,
I was just raising the fact that not using the manufacturers recommended pressure is not to be recommended :D
The Prius, assuming their claims are accurate, is not a particularly heavy car. From 1300 to 1325kg depending on options, or about 2900lb in the old money. About the same as a VW Golf diesel.
It is a bit naughty of Toyota in the USA to undertyre the car - I assume from other threads that bigger wheels are fitted in the UK, mind you in the US the car is MUCH cheaper, as usual. I would still have expected them to recommend the optimum pressures.
 

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I always thought that recommendations were just that...recommendations...much the same way MSRP is a recommendation, and the dealer choses mark-up/mark-down from there...
 

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BIF said:
The Prius is approximately 3,800 pounds. With me in it, let's just call it two tons.
BIF,
I think the Prius weighs around 2900 lbs. empty. Unless you are an exceptionally large person your estimation of the combined weight looks to be a little on the high side.

Drive happy,
Moo :)
 

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f1eng said:
It is a bit naughty of Toyota in the USA to undertyre the car - I assume from other threads that bigger wheels are fitted in the UK
Bigger wheels are fitted in the UK and elsewhere. It's a shame that bigger wheels aren't at least offered here, but I know some dealers are replacing the stock wheels with 17" wheels, which is perhaps a bit much. :)

Toyota really did skimp with the default US tires... Still, they're better than the previous gen Prius.
 

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Discussion Starter #18
moocatdog said:
...I think the Prius weighs around 2900 lbs. empty.
I just checked my door sticker, and here's what it says:

GVWR: 3795 LB
GAWR: 2335 LB
RR: 2250LB

I've always been under the impression that the first number, GVWR, is the total weight of the car. That's where I get the "two-ton" figure. Please correct me if I have misread something.

The Toyota website lists "curb weight" at 2890 LBS. I cannot explain the discrepancy, but for the tire-discussion, I'll be conservative and assume that the biggest number, GVWR on my door sticker, is the weight of the car.

I also checked the tire numbers. Here they are:

Tire sidewall: 44 PSI Max
Door jamb sticker: 35/33 PSI (front/rear)
Valve Stem labels: 32 PSI


Talk about a "mixed message!" With so many different recommendations being made by so many "engineers" who apparently don't talk to each other very often, I figured that "the truth is probably somewhere in between." So I made a choice of 41.5 PSI Front/40 PSI Rear.

Some of us at my place of work think that the car manufacturer decreases recommended tire pressures not so much for safety reasons, but for ride-softness and marketing reasons. After all, I live in the US, land of big cushy couches, Lincolns, and SUVs.....

Given all of this, I still believe that my choice is a reasonable trade-off between ride quality and mileage. It's also a reasonable compromise in terms of safety, because I'm not "at max", and I don't often drive at warp speeds.

My first post here said that the tires each had less than 30 PSI. After having put more air in them, the difference in ride quality has been negligible for me. However, here's the real eye-opener: For this tank, I'm getting an extra 9 to 11 miles farther on each gallon of gasoline. That's an extra 100-120 miles per tank! I'm not driving any differently; it's just that the car isn't spending so much time and effort fighting tire friction!

I hope somebody with automobile design and/or dealer marketing experience can shed some light on the possibility that dealers and manufacturers decrease tire pressure for ride quality reasons.

Thanks!
 

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The 3795 weight is a maximum which should never be exceeded, eg hauling bags of cement or full up for a camping holiday!
The car weighs just under 3000lb plus pasenger weight plus the weight of an luggage and fuel.
The tyre pressure marked on the tyre is the absolute highest that should be used safely on that tyre construction. Remember the tyre manufacturer does not know all the vehicles customers may fit their tyre to, so have to quote the maximum.
The CORRECT pressures for your car are the ones on the door jamb.
Running higher than this will reduce grip making skidding more likely and increasing braking distances. Not good if you need to avoid an accident.
Many may be prepared to take that risk to save a bit on fuel. I would not personally, but I deal with car handling every day, it is part of my job.
 

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Discussion Starter #20
f1eng said:
...The car weighs just under 3000lb plus pasenger weight plus the weight of an luggage and fuel.
How do you get this figure? None of the other figures are even close to this.

...Many may be prepared to take that risk to save a bit on fuel...
If you are correct, and my car weighs nearly a thousand pounds less than I originally thought, then tire overinflation is EVEN LESS of a concern of mine. After all, underinflation and foreign-object puncture are the two most frequent causes of tire failure. I would venture to say that the third is probably excessive wear (old tires).

I now believe that 32 is most definitely underinflated, and 35/33 is probably also questionable.
 
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