42/40 Air pressue unsafe???

Thanks Karl. Think that needed to be pointed out. You don't want to compromise safety for a 1-2mpg advantage. I keep the tires at 35/33 and am perfectly happy with the 45mpg average I have been getting.

 

No, it is not unsafe.

The Tires do not decrease their contact patch at 42 PSI.

In fact, if anything, it is safer, particularly in wet or slushy conditions.

You are talking about extreme conditions of over-inflation that don't occur with modern radials in normal circumstances.

If you believe everything an auto manufacturer tells you about what is the 'recommended' inflation of tires for a given vehicle, then you believe that Ford was right in their recommendations for the Exploder which lead to those unnecessary deaths.

Your credentials, while impressive, are not necessarily any better than others on this forum. Prius users are an extraordinarily educated and diverse bunch.

I for one, as an engineer, never take the word of self-proclaimed 'experts' without doing my own research, and my research contracticts your blanket generalizations.

The analyses and statistics showing that cornering Gs increase and braking distances decrease in almost all cases of inflation past the manufactureres (comfort oriented) recommendations have been posted here many times.


Aside from the physics and mathematics involved, I have found through experience that inflating past the recommendations of (vehicle) manufacturers almost always results in a more responsive and better handling vehicle.

The mileage is a side benefit. I haven't run at (vehicle) manufacturer specs since shortly after radials came out.

Of course, no one is saying that you should exceed (tire) manufacturer specs, but don't try to sell that old canard about the door sill number having anything to do with the auto manufacturers concern about your safety.

 

I'm not a tire expert and I don't play one on the internet. But there are those that do play tire expert because they have been using tires on a Prius for years. Their personal observations of longer tire wear (compared to what I am not sure of) and better MPG make them tire experts.

If you really want an arguemt over this issue, go to toyota-prius group on the Yahoo Groups.

 

I'm not an EXPERT either, but my Prius teck agress with 42/40. At Toyota's recommended pressure tires wear prematurely on the outer edges. This to me shows that the tires aplly most of the pressure to the road with the outside edges. At 42/40 tires wear evenly to me showing the pressure the tires apply to the road is evenly distrubuted. Results are increase tire life and increse mpg.

 

Absolutely.

Maybe we can get to a point that all can agree here, though from experience I doubt it

1) Underinflation is bad. If your tires are sufficiently under-inflated you will see a specific wear pattern, and it can have safety ramifications.

2) Overinflation is bad. If your tires are sufficiently over-inflated you will see a specific wear pattern, and it can have safety ramifications.

3) Vehicle Manufacturers set their 'recommended' inflation numbers for any number of reasons, only one of which may or may not be safety.

4) Sometimes the Vehicle Manufacturers get it wrong. Witness the Ford Explorer debacle.

Now, with the Gen 1 Prius, it is pretty universally accepted that Toyota got it wrong. Using their inflation numbers ate tires in 10K or less showing classic under-inflation wear patterns.

5) If your tires are properly inflated, you will not see either over or under-inflation wear patterns.

6) There is a range of inflations that fall within the bounds of not causing abnormal wear, and could thus be considered 'normal' inflation.

There will of course be much arguement and gnashing of teeth over whether higher or lower inflation (within the acceptable range) is better for what conditions and circumstances. And, in fact, there are indeed differences. The inflation that will minimize hyrdoplaning is different from the inflation that will work best on glare ice. You would also inflate your tires differently if you were going to a Drag Strip than if you were going to a Road Course. No arguement there.

But, blind acceptance of the manufacturers 'recommendation' for inflation is putting on blinkers. The manufacturer has their own agenda, and it may or may not coincide with yours.

It's just like oil changes every 3.000 miles. Just because a for profit corporation suggests it does not mean it's 'right' for all circumstances.

 

While I think this site is great for Prius information, I think some of the Prius owners here seem to think they are some sort of gurus and above the laws of physics.

As far as physics goes, and I don't pretend to be an expert, but I do know some basics, it does not make sense that an over inflated tire will brake better. There is less contact with the road. Am I missing something here? I could be wrong, but from what I have learned about friction, this statement has me confused. yes it will decrease resistance and bring up the mileage (I'm with you there), but how does it INCREASE grip on the road?

Plus when temps are in the 100's and you travel to higher altitudes I don't think you want your tires to be inflated to the maximum pressure allowed (that nasty physics again). I do this frequently and don't feel comfortable with my tires inflated to the maximum.

Now if you are all arguing about the manufacturers not posting the correct presssures, maybe you have an argument. But I would think the tire company would work very hard to find the safest pressure for the tire with maximum performance...why wouldn't they? And yes comfort is a factor too. I personally don't like being jarred around.

I bet you guys arguing this point are the same ones running out of gas just to see how much you can eeek out. Whatever makes you happy I suppose. As long as you don't hurt anyone...which could happen if a kid runs out in front of you and there is a bit of gravel on the road.

OK guys now have a heyday and rip me apart. I never got better than a B in college physics. So maybe there is something I missed?

 

Then junk the OEM tires and get something like Goodyear TripleTreds. I can now accelerate into turns without worrying about traction control kicking in, and the MPG loss isn't all that awful.

However, I'm still running the tires at whatever pressure the Goodyear dealer set them at, and will get back to 42/40 one of these days. Maybe that'll add a few MPGs...

 

opinions are like...

I think its really pretty simple in the end. Inflate your tires as you will and watch tread wear. I keep mine at Toyo spec'd until I notice something odd in the wear. If I don't it will stay where it is until I get new tires. At 10000 miles, I've yet to notice anything.

 

Excellent contribution, Karl! Especially the invitation a look at documented facts. I notice that many posts offer undocumented personal observations, but scientifically designed and controlled tests rarely are quoted. Those personal observations are not without value, and any scientist will look at both, and even insist on duplicating the research to verify data and conclusions. This forum is great for stimulating both entertainment and education.

Dick Neufeld

 

I think Karl Schmidt brought up many good points that should be addressed.

First of all, I almost never trust "seat of your pants" testing. Empirical data is all I'm interested in. So unless we can instrument a 2004/2005 Prius and try out many different tires and inflation pressures, especially over different road surfaces and water/snow, it's all puffery.

For example, a few years back there was a lot of hype over these sophisticated ceramic brake pads. There were also some complaints from various State Patrol departments about poor overall braking performance of some aftermarket pads.

In particular, some smaller police drivers could *not* exert enough brake pedal pressure, even with both feet on the pedal, to even get the ABS to kick in. When the brakes were hot, they worked fine, when they were cold, they were dangerous.

The National Institute of Justice had the National Law Enforcement and Corrections Technology Center perform an instrumented test on "typical" police vehicles of the day - 1996: Chevy Caprice RWD and Ford Crown Vic.

http://www.nlectc.org

Their testing confirmed the reports: most aftermarket "performance" pads were ineffective, even *dangerous*, when cold. The "performance driver" enthusiast would probably throw up all over the Empirical results, but they were verified.

Long story short: no one tire will satisfy every driving condition, and no one inflation pressure will be prudent under every operating condition.

For example: Some folks have reported "excellent" results running all season tires, such as the Integrity, on snow and ice. They MUST be joking! I've gotten my Prius STUCK trying to go up a slight parking ramp with all season tires.

OTOH with my Dunlop Graspic DS-2 studless snow tires, I can handle snow under 12 inches, and especially ice, far better than any 4x4 with all season tires. There is no difference in city fuel economy, maybe 3 MPG on the highway. But they aren't safe in summer so I take them off the first warm stretch.

I have found that my 2004 Prius has a very harsh suspension; that is, it has poor compliance over washboardy roads, city potholes, and city expansion joints. Up to a certain point, a rock-hard suspension may help "handling" at the expense of ride.

Put a car with a rock-hard suspension around a corner with bumps, especially a washboardy corner, and that car will probably spin out or leave the road. Yet a traditional big car with a soft floaty suspension will have no problems.

To enhance fuel economy, I have tried up to the maximum 44 psi cold inflation limit, and there was no difference in my city driving under 80 km/h. On the Trans Canada at 100 km/h (62 MPH), there was usually no difference in MPG. A couple of times it appeared I gained around 2.5 MPG according to the MFD calculated average.

At 44 psi the ride became harsh and downright punishing. IMHO unless your pavement is absolutely smooth, it's not worth shaking the car to pieces. Also, at that high an inflation pressure, the car is much more likely to skid or slide over bumps.

I personally leave my tires at the Toyota recommended inflation with no wear issues at all over 17,000km. My Michelin dealer claims with a car as light as the Prius, 30 psi would be just fine too.

If I really want to gain 1-3 MPG, I'll just slow down 4 MPH. For example, on the Trans Canada if I set the cruise at 90 km/h instead of 100 km/h, it takes me an extra 7 minutes to get to my hobby farm.

Big deal.

Instead of an MFD calculated average of 5.4 Litres per 100km, I'll get an MFD average of 4.2 Litres per 100km. That's going from 52 MPG to 67 MPG Imperial gallon. All it "costs" me is 7 minutes.

Jay

 

Well I don't have data and as I am fond of quoting my old professor " In the absence of data you can have any opinion you wish" I will insert my observations for someone who wishes to add it to a meta analysis. Do not fill tires on a Ford Explorer to the max it gets really squirrelly at high way speeds. I know I have tried. The Prius at 42 - 40 drives well as far as I can tell and I get 3-5 mpg. The ride is not too rough as well. Anecdotal information fine but it is not data it is a story.

 

Well, it is true that the maximum load stamped on the tire sidewall is rated at the maximum cold inflation pressure.

For example: my Michelin Harmony "all-season" tires in stock P185 65R 15 size are rated to carry 1,168 lbs at 44 psi. My Dunlop Graspic DS-2 studless winter tires are rated 1,235 lbs at 44 psi.

According to the brochure, the Prius has a curb weight of 2,943 lbs and a GVW of 3,795. At this point, without a weigh scale check of the front, the rear, and both, we have to guess and make a lot of assumptions.

If we just guess an equal weight distribution to keep it simple, at maximum GVW around 950 lbs per tire. We would have to contact Michelin or another willing tire company to determine at what inflation and - more importantly - road speed the tire will safely carry 950 lbs.

However, we do have to assume the Prius has a front weight bias due to being a FWD car. Say 70/30. So at 3,795 lbs maximum GVW, that would be 2,656 lbs over the front axle and 1,138 lbs over the rear axle.

So the front tires would need to handle 1,328 lbs each and the rear tires would only have to handle 569 lbs each. Neither my Michelin "all-season" tires nor my Dunlop winter tires can safely handle the front weight at maximum GVW, that is if my assumptions are correct.

However, can we still assume that weight bias applies with load distributed evenly? That is, do 2 passengers in the rear seat shift the bias closer to 50/50? Does the traditional FWD car front-heavy weight bias only apply with just the driver in the car? And can we also assume equal left/right weight distribution empty and at maximum GVW?

See, without proper testing to gather empirical data we can only guess. The word "assume" can be broken into nice person/u/me and that pretty much spells it out.

 

From the Goodyear website:

(a)Tire Size
(b)Service Description
©Sidewall
(d)Approved Rim Width
(e)Meas. Rim Width
(f)Section Width
(g)O.D.
(h)Tread Width
(i)Max. Load

(a)P185/65R15 (b)86S ©B03 (d)5.0-6.5 (e)5.5 (f)7.4 (g)24.4 (h)6.4 (i)1,168 @ 35 (Integrity)

(a)P195/65R15 (b)89T ©VSB (d)5.5-7.0 (e)6 (f)7.9 (g)25 (h)6.1 (i)1,279 @ 44 (TripleTred)

Unless the OEM tires are special Integritys that we need to reorder only from Toyota, either we're in trouble buying anything Goodyear can sell us, or the load from a Prius is distributed well enough to get by.

Take your pick, but I like the TripleTreds at 44psi myself... :mrgreen:

 

Another good question that can only be answered by properly testing scenarios. If one assumes a perfectly equal weight distribution at maximum GVW, then each tire will "see" around 950 lbs.

Remember: assume = nice person/u/me

I guess we just have to trust Toyota on this until somebody can do testing to prove otherwise. I'm wondering if the F/R inflation bias is more for ride comfort or for some handling issue?

That is, everything else equal, a tire at lower inflation will stick a bit better to the road. Perhaps Toyota wants understeer/plow instead of oversteer or the tail swinging out. Again, that's just a guess.

 

[font=Comic Sans MS:22487f175d]If you look at the jacking diagram in the owner's manual, it indicates that the Prius' center of weight is in the middle of the cabin. just about in line with the driver's (and passenger's) rear, so the car is balanced front to back. The motor, right over the front tires, puts more load on them on a bump, so the front tires get 2 PSI to help compensate.

I agree that an instrumented test would be ideal. In the meantime, I'll watch the treadwear on my lightly loaded (higher MPG) car as a guide to proper inflation. [/font:22487f175d]