Tire pressure

[F8L]

Originally Posted by eestlane

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.

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.

[mingoglia]

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....

[eestlane]

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!

[mingoglia]

Originally Posted by eestlane

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.

[F8L]

Originally Posted by eestlane

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 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

[DLHDMRH]

Originally Posted by eestlane

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.

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?

[Flying White Dutchman]

Originally Posted by eestlane

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!

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

[Rest]

Originally Posted by eestlane

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:

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.

[a priori]

Originally Posted by eestlane

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.

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.

[diamondlarry]

Originally Posted by F8L

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

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.