Tire Pressure

I suppose the optimum tire pressures would be both subjective and objective. Play around with each combination for at least a couple of months then make the decision for a prolongued period. Taking for granted that the oil overfill problem is taken care of pretty well leaves comfort/control/tire wear and mpg.
We've made our decision on 42 front/ 40 rear. Perhaps an infinitesimal more bump than lower pressures but thats to be expected.... still a very comfortable ride. Control "seemed" somewhat improved. Tire wear will have to be determined after the prolonged test period and then only by comparison with someone who has used a different pressure over roughly the same test period.
My MPG comparison over 4 tankfills during relatively same temperature and road conditions are:
Tank #15 46.65mpg US
Tank #16 44.86mpg US
Upped tire pressure from 35 front/ 33 rear to 42 front/ 40 rear
Tank #17 47.46mpg US
Tank #18 53.61mpg US

Vehicle total mileage 6974.5 and just over 1 year in operation.
I'll stick with this until my data tells me differently. Heck,our darling isn't even broken in yet.

 

Someone mentioned hydroplaning...

The 'rule of thumb' formula for dynamic hydroplaning is 10 * squareroot of tire pressure in psi = hydroplaning speed in mph.

A more accurate formula is 7.95 * sqrt (PSI * contact patch width / contact patch length) = MPH

Viscous hydroplaning i.e. water+oil, etc. can happen at lower speeds.

 

Yea.. or nice big puddles you hit in the dark going 40MPH without warning!

Based on what I've read so far... going up to 44 ish or higher tends to wear the center out faster.... giving less traction and handling in dry normal conditions although prob a tad better against hydroplaning. Any tire that wears noticably in the center, will greatly lessen the life of the tire due to more weight of the car knawing against the contact surface of the tire with the pavement.

Yet dropping to the lower pressures (<30) may give a nice soft comfortable ride with nice dry pavement traction, but kill MPG as well as wear the edges of the tires?

It sounds like the "balance" I will prob try for is:

This sounds like a good balance.. I know some tires are not supposed to "bulge" with extra tire pressures and this characteristic will probrably vary from type of tire to tire.
But any tire will bulge if you go crazy over its limits.

So each person will have to measure his tread depth after 10000 miles or so to determine if he has picked the right pressure for his tire....
And then stay on top of his pressure in order to have accurate analysis.

I'm sure we can go above the rated pressure without bulge as long as it doesn't get out of hand...
Personally.. I don't want my tires to bulge!.... it shortens the life and if I"m that concerned about mileage.. why not just get tires that are not as wide in the first place to accomplish the same thing and not kill life of the tire in the process?

So for me at least, I will be attempting to see how far I can push the envelope with higher tire pressures to attain maximum MPG "without" tire bulge and not prematurely wear the center tread away . I'm guessing that this "majic" pressure will also be a good comprimise for a good comfortable ride without feeling like I have stage coach wheels!.. :lol:

I"m not going to sweat the hydroplaining issue.. I think tire pressures should not be centered around what gives the least hydroplaining at the expense of everything else. Like I said... put on motorcycle tires if thats all that matters right?

Most people who purchase a prius should be good drivers and know the limits of the tires and stay below that, and watch the road conditions and try not to be stupid!

I'm looking forward to being able to monitor my air pressure with the car display.. I hope that works well!

 

I didn't think that the actual air pressure will be able to be monitored on the car display? I thought that it will only show if there is a rapid pressure drop in a tire?

 

Without disputing you, I have yet to read of any excess center tread wear on Prius tires at any pressure. Without a doubt this is a commonly held view, and indeed was expressed by Toyota's Dave Hermance in San Diego last month.

Back in the old days of 2001 Prius, I found that the OE tires gave even wear 42f/40r. By coincidence, the same result with Goodyear Allegra 175/65R14 replacements. I have just installed Nokian i3 185/65R14 tires, so too early to comment on those. For the moment I have them at 40/38. My confidence in tread wear rates is based on >1000 tread depth measurements through time with a digital depth gauge. However, no personal experience with the new model Prius.

Further, it is my understanding that the geometry of under-tread belts in radial tires would make it unlikely that realistic pressures could "balloon" the tires.

Always hoping to understand these issues better. Obviously tread wear is one of my hobbies. My goals are maximum tire performance and (secondarily) maximum tire life, but they would both seem to be served by even wear across the contact patches. For the second goal, my tire rotation is based on observed wear rates at the four positions. There are probably better hobbies...

 

Thanks Wilco for using physics to clarify my point. Since people are more likely to under-inflate or not check when air pressure falls, there is less worry over over-inflation. I can't prove it, but these "increased crashes" when it rains, particularly on the interstates, are most likely caused by hydroplaning due to low pressure on one or more tires.

 

Records, yes. 18-24 tread measurements/tire, about every 4k miles. It started out yrs ago because the OE tires had such a bad reputation then. Continued into the 2nd set of tires, because they offered less than the official load-bearing capacity. Now even though I switched to tires with much higher load rating, the habit sees fixed in place. Maybe after I settle on their optimum pressure I will seek counselling.

175/65R14 was the stock size for the classic Prius sedan; worldwide I believe. The new Prius (of course) has 15 inch wheels in the US and 16 in Europe (at least).

Much Prius Chatting about alternative tires. Not for me to summarize that, but the OE Goodyear Integrity generates a wide range of opinions, GY Comforttread a popular replacement at about $80, and Michelin and Nokian have several for about $100. There are also Falken and Pirelli replacements under Prius.

 

Yes I have read/heard that also and from good sources too, unfortunately I can't find it now. Also, the wear people on this site have reported tends to bear that out. i.e. even people who use higher pressures 40 to 44 psig have reported outside tread wear is higher with the OEM tires, why I don't know, but with bias ply tires it would be the center wearing faster. I also beleive that increasing the pressure in a radial ply tire doesn't increase it's circumference by very much, as it does in a bias ply tire.

What I have noticed, anecdotal only, is that the pressure I carry in radial ply tires, as long as it is high enough to prevent over temprature and below the recommended maximum, has little effect on tread wear. But it does have a pretty big effect on mileage even on cars other that the Prius. When I increased the pressure in my Accord's tires from 35 to 42 lbs my highway mileage went from 29 or 30 mpg to 32 or 33 mpg.

 

This is of course an extreme exaggeration of what a car experiences.. but I am always amazed at how much tremendous weight, for how fast, for how cheap, a train can carry over such vast distances for so economically! Steel on steel.. now thats tire!... a bit bumpy though! :lol:

 

Thanks for the info.. so It sounds like there is really no upper limit to worry about bulge as long as you stay within the specs of the tire max. "provided their radial"

I would certainly think however if you did want to run very high pressure in the name of MPG, you would want a softer tire "like goodyear maybe" or you could really go skidding without traction even on dry pavement?.... let alone wet?

So we are in a deleima?... too high of a pressue looses too much traction for safety, but going softer offsets the MPG gained with the extra pressure because softer tires increase MPG I believe?
Its all about traction... the more you have it.. the less MPG you have!

I don't know if you can have both or not??? I would think higher quality rubber would give you the suppleness to grab the pavement without being ate away by that pavement too soon? In other words a high mileage rated tire that has good rubber?

But even with the higher quality tires... its hard to tell if high mileage rating is due to rubber quality or just merely harder cheap quality rubber? The latter would be bad for traction as well?

Too many variables.. it makes me dizzy!..... Sorry for letting my thoughts ramble.... kind of a brainstorming session!

 

:huh: :blink: <_< <_< OH well...

 

Overinflated tires increase both the wear on the tire (central treads) and increased rate of blowouts. It should decrease the safety when it comes to braking (increased braking distance). I think, also, that tire pressure is being recorded in two different areas of the vehicle in the event of a crash. In other words, if you felt like your tires were the problem in a crash and went after Toyota in a lawsuit, they would have a record of your overinflated tires. I don't think the above justifies the increased MPG!

 

I think we should always put Safety before MPG, driving a Prius or not.

Overinflated tire will reduce the size of its footprint in contact with the road, thus reduce rolling resistance to yield better MPG. However, the tire will also has less traction due to the reduced footprint as well, thus decrease safety esp under wet condition. And it could be damaged more easily when encountering potholes or debris.

 

If you are driving any car in wet weather or slick road conditions, slowing down and giving yourself more distance from the car in front of you has a much, much greater impact than driving with tires at 32 PSI vs 42 PSI.

Basically, you can control all 3 of these, but if you emphasize your caution on speed and following distance you won't need to worry about tire pressure.

 

Are you aware that you are responding to a thread that hadn't been active in nearly 2 and a half years? Anyhow, I doubt the reliability of your statements. Do you have links to reliable independent sources that back those statements up, or is it just what you believe because it's what you've heard.

 

My experience is that higher tyre pressures give more responsive handling due to less sidewall flex, there is less heat buildup in the tyre, a major cause of tyre failure, and aquaplaning should be reduced if the contact patch is reduced because the mass of the vehicle will have an easier job pushing a smaller area of tyre through the water.

It is also my understanding that friction between two items is a product of the force applied and the coefficient of friction between the 2 surfaces, area has no bearing. There is an improvement in grip using wider tyres because it allows the tyre to grip sticky parts of the road if part of the tyre is on a slippery part. Racing slicks have very soft tread which moulds around the aggregate in the road's surface and the compounds in the tread get sticky when hot.

 

Danny Hamilton: I am new to "forums" and was not aware of the 2 year old date of this thread. In fact, it took me this long to find out where I wrote my post! I believe what I have read, and I have read several articles over the years regarding tire safety, all warning of the dangers of under-and-over inflation. You can check it out further yourself if you doubt my information, and here are but two articles: Your Tires, Your Money, Your Safety - Coast Tire & Auto Service (it has a nice picture) and www.articleclick.com/Article/The-Im...-Tire-Pressure-For-Better-Fuel-Efficiency-and Optimal-Handling/220. It seems obvious that overinflating a tire "rounds" out the tire, thus putting less tread in contact with the road, and thereby affecting stopping distances and safety.
As for what I said about the tire pressure being recorded, that was ENTIRELY WRONG! The EDRs record several items in the event of a crash, but they do not record tire pressure. I remembered incorrectly.

 

My skinny road bike tires carry 125-140 psi rating, and that's where I run them. As a kid, I stupidly blew out a 10-speed tire (cheapo) at somewhere in the 150-220 range, not realizing what small volume they held. My mountain bike slicks are rated 80; I run them lower on rough surface rides.

Anyway, I fully appreciate the risk of catastrophic failure of tires. But c'mon, there's no way anything like 44 psi (in tires so rated) will "overinflate" one bit. No center wear, no loss of traction, no extra road hazard vulnerability. If you can stand the NVH, tire sidewall is the place to be (unless you are comfortable going above that). In fact, better wear, traction, etc., imho.

 

So what is the consensus on the Touring Edition psi's. I am currently running 42/40 and they seem fine to me.