Tire pressure

I do believe the only drawback is ride comfort, which is the main reason toyota recommendations are conservative. Higher PSI will benefit you more than hurt you, IMO. I have not raised the PSI myself yet on my 2 week old prius, but plan to on the next fill up. I plan to try 40front/38 rear.

 

Hey Lauris, prauks Riga! Don't listen to all of the "high pressure nuts" on this forum. I tried it higth and I tried it low. Fuel economy is improved very little versus the rougher ride, more damage to suspension components due to excessive shock, poorer handling in crosswinds, tires being more susceptible to road hazards (some gravel roads in Latvia, just like in my birthplace Estonia), etc. Toyota engineers ran many tests to recommend the proper tire pressure for this car. After all, the main purpose of the Prius is to get good gas mileage. So why would they throw away some of that good mileage just to get a little bit of a softer ride?

 

Sorry .... gonna have to quote in English measurements of psi.

Anywho ........ I've done a lot of experimenting with tire pressure. I've gone as high as 44/42, and as low as 33/35 (Toyota's). After all of this, I've found 38/36 to be about perfect.

It's really a trade-off between comfort and FE. However, I have found that if my fronts are 40+ psi, the auto gets a bit drifty on the highway at speed (75+). Therefore, safety also plays a role.

My advice is to try out how different tire pressure feels to you .... then make an educated decision based on that feel, and what you've read on this board.

... Brad

 

Care to show us "nuts" your scientific testing of high pressure vs low pressure?

 

Dear F8, as a professional test engineer I am quite capable of running various tire pressures and comparing long-term mpg differences, which I did. I found no mpg increase from higher pressures that were worth the negatives I stated. An increase of 2 mpg is not a significant percentage of the overall mileage, and not worth the disadvantages. Also, as an engineer, I somewhat resent the amateurs on this site claiming outrageous pressures (50 psi) as being "safe", when Toyota's professional engineers recommend 35/33 psi. You think they designed a very good car and then arbitrarily assigned the recommended pressure willy-nilly: "Oh, let's see now, what should we recommend as tire pressure? Oh well, lets say 35/33 and see if the customers accept that value"? Think again.

 

Children...Children...You are blowing this out of proportion. As long as you say within the limits posted on the tires itself, safety is not an issue. Both of you are right. It can be argued the same way as to what is the best way to drive a Prius; for enjoyment or for max. MPG. I find this and other posting interesting if not educational. Let the buyer/driver decide. I thank you for your comments and discussion. Bottom line; both of you are right.

 

There is a retired tire engineer that has some very good data and has answered a lot of questions on another forum. Here is the link to his info.

 

Here is a link to an article at officer.com on tire pressure. I hope these articles helps clarify what some of us "nuts" are doing. I have the Bridgestones on our Touring and it will glide, coast, roll and propel itself much farther with at least the sidewall rating of 51 psi than at the paltry 30's of the doorsill placard. Super highway mode can be engaged much easier and farther as well.

 

You hear that Evan? those of us with 60mpg - 600+ mile per tankfull are not safe. Those of us who've been running 48-50psi and higher are dangerous. toung&cheek done. Now, on to serious-ness.

Example #1 Our Lexus 400h hybrid came with a tag stating 34psi. Most 400h owners ended up with balding tires within 12K miles.

Example #2 My electrical engineer father in law always designed his projects with circuit boards with an empty spot. He said the reason was because his fellow engineers were often "idiots who'd forget critical things, and the bald spot allowed for intigration of the item(s) they'd left out".

In case the point is not glaring, what ever your craft, it doesn't mean "THE-authority" is always authority. Sometimes one has to reverse engineer, so that proven results match what the slide rule says, in stead of the other way around.

That said, no one running 50lbs here on PC is hard driving, diving through gravel hairpins, because we know soft tires on a BMW will respond differently than our Prius. You have to be senseable take all the variables into account. 50lb psi prius drivers drive slower, and more anticipatory.

 

I asked to see the data, that's all. Can you provide it or at least explain to us your methods and procedures? Even engineers forgot to make corrections for other parameters and to even include certain conditions which could effect the overall results. This is why I asked for the data. No one is perfect.

 

Wow, this thread is turning into a big swinging <insert slang term for male reproductive member here> contest. I too noticed a measurable increase in MPG when bumping up to 42/40. This increase would probably not be measurable in most cars, but since the Prius gets such great fuel economy to begin with the approximately 8-9% in the increased pressure translates into whole numbers as opposed to a fraction in a vehicle that let's say gets 10mpg. 3-4 MPG sounds like a lot, but in the grand scheme of things, relative to 50mpg, it's small.... but certainly measurable and worth it if it doesn't reduce tire service life. Time and time again it's been proven that folks can get even wear at 42/40 (on no Touring Prius'... I haven't followed tire pressures as closely on Touring models).

Life is a compromise. I believe Toyota will recommend the lowest air pressure possible that allows the contact patch they're looking for. This is because the car will ride better (softer, quieter, etc). I believe the 42/40 provides the same contact patch... it's just the higher end of the pressure range. One day I may consider testing this... the contact patch can easily be checked at home with a bit of chalk and some time....

 

OK, OK, I didn't mean to start a fuss. But I do think that we shouldn't lead newbies to think that pressures at or above the sidewall max is a good thing for everyone. Some people are recommending such pressures, and aren't stating any caveats to look out for. They are free to do as they wish, but do not advise others to do likewise, just because you are not liable if something goes wrong. Also, I have seen no mention that the Toyota recommended 35/33 is good even at full vechicle gross load, since a higher value for a full load isn't stated. For folks driving alone, or two people with no baggage, that is already somewhat higher pressure than is needed.

As for data, I drive the same routes day after day. I increased pressures every month by 2 psi all-around, reset the accumulated mfd mpg, and noted the gain in mpg. I wound up at 41/39. Sure, the mpg increased somewhat, but not enough for me to be worth the negatives. People complain about their Prius wandering in crosswinds or when being passed by a truck. Was their tire pressure too high? Maybe. What about the life of the shocks and bushings, etc.? What about your loose dental fillings? I drive Yokohama Avid TRZ's, which are quite a good tire according to CR, and I agree. Very good ride, super hydroplaning resistance, good tread life, and reasonable price. They come in the 195/60R15 size, and even handle quite well! And a good day to all!

 

I agree. I should have clarified that I believe 42/40 is acceptable but I would in no way recommend exceeding the sidewall max. Heck, I'm in Arizona so I'm very careful about air pressure due to the extreme heat, higher freeway speeds, and longer period of time at higher speeds. I'm also a bit more anal about air pressure due to towing a large RV that likes to eat trailer tires. In fact most of the time I have my IR temperature gauge and check tire temperature when I stop. I'm a bit fanatical about tire pressure. In fact I won't even check my tire pressure after the sun is up and had a chance to beat on one side of the car more than the other.

 

I have not seen anyone advocating going above the max tire pressure. I do see people stating that they run high pressures, myself included but we do not state people should run higher than 44psi.

Did you try to run higher pressures than 41/39? Most of us that run high pressures are in the 55-70psi range. If you did not try to go that high then I'm not sure your conclusion should include any pressures higher than 41/39 because there was no data obtained in those regions. If there was a small gain at 41/39 then could the gain have been larger for say 70/68psi or is there a point of dimishing returns? We need to get someone to collect data and put it into a spreadsheet for us. *yells* Hey Bob.........

Once we have some quantifiable "negatives" I can take thos einto consideration and maybe modify my tire pressures but all we have right now are opinions and some outright falsehoods. My car does not wander any more now than it did when tire pressures were stock. I do not pick up more rocks and gravel than I did before nor have I seen any data on shock or bushings wearing sooner under higher tire pressures. True some of this makes sense but there is no data so it's hard to say anything definatively. People told me my tires would wear out in the center if I ran higher pressures and that didn't happen either (based on 185/65/15 Integritys). The ride is rougher, I can't argue that one. I actually prefer that to the soft "marshmellow" ride I observed when the pressures were low.

I respect your opinion as an engineer (not sure what you specialize in though) but I would like to see numbers or evidence before I flame anyone for their opinions. Have a great day too

 

Hopefully my wife and I will be receiving our new 09 Prius in the next month or so. I have always been very particular about keeping proper tire pressure. I see you say the recommended psi for the Prius is 35/33. Are you recommending different pressure for the front tires as opposed to the rear??? If so.....why?

 

lose dental fillings?
life of the shocks ?

well that a bit to much for me.
when your dentals really fall out then there something wrong wit your car nog the presure in your tire
and if the shocks can not handle the higher tire pressure well what wil happen with normal day driving? OMG then it wil break...

i do 4 Bar on my tire and i am fine with it
maybe the roads in your area are really BAD

 

And as an egnineer you would also know Toyota had to make a compromise between safety, comfort and fuel mileage. And you would know the burst rating on tires is well beyond the tire's max rating.

I run 50psi front and 48psi rear on my 2007 Prius. I also had run 50psi all around, on a 2001 Honda Insight I used to own. No problems at all.

 

I don't mean to offend, but I'll make the observation here that the decision to set 35/33 as a recommended tire pressure was no more made by professional engineers than was the max pressure rating of 44 (on the stock Integrities).

I am suggesting that the engineers had input into the decision-making process. This is no little input, as it likely was critical to the decision-makers to find out what the engineers say about maximums and risks. Still, there are other factors. They must consider the quality control at the factory. They must consider the cost of recalls or loss of "brand" value for failures. Nearly every extra consideration will cause the decision-makers (upon advice of counsel) to be more and more conservative in setting the recommended pressure and the max pressure.

The fact is that the costliest problem is that one that happens when the claimant is operating within the recommended guidelines. You cannot, therefore, set a recommended number anywhere near your 95% certainty point -- not at 99.5% certainty, either.

I have no problem going right up to the max sidewall number noted by the manufacturer. This is not the burst pressure! Goodyear is not going to set a max number on a consumer tire only because it has a 99.5% certainty the tire will not blow at that pressure. NO! They have set that pressure because of giving creedance to many other factors.

Why the Toyota decision-makers have gone even lower is not one I can call. I suppose they were aiming more for comfort and surer handling. They know that hypermilers are going to challenge each number, so they will build in extra safety factors.

 

I would be one of those you are referring to. My son and I are both running psi's that are even a bit higher than the upper end you mentioned and have had no problems of any kind, handling or wear. I have 22K+ on my '07.