Thank you for posting that. I've heard some suggest using pressures well over the max on sidewall. I've never thought that was a good idea.

I've been running 4 PSI over front and rear.

This is a really good post. Very important to consider.

i figured this to be the natural reaction of over inflating. tires need to flex in order to grab the road.. more grab equals more grip. more pressure equals less grip... shrug? i stopped paying attention to my tires psi. i fill them every couple months.. that's about it. (then again.. i'm happy getting 38 to 40mpg)

I don't agree, but I'm not trying to get people to wear out there tyres faster so I can sell more tyres.
My car handles better with more air in the tyres.

It's a valid point about the tyre manufacturers, but what's in it for the car manufacturers to use the lower tyre pressures, if the handling and stopping distances could be improved just by pumping up the tyres some more?

Do the tyre manufacturers wield (wheeled - tee hee) that much power over the car makers as to dictate the psi that goes on the door placard?

Hi All,

Remember these comments are generalised for the wide variety of vehicles and conditions.

I find this quote specifically telling with regards to the situation with the 2nd Generation Prius:

"
According to Michelin, "The first thing to understand is the tire's interaction with the pavement is through contact-footprint forces, which are significantly affected by inflation pressure. Altering these contact-footprint forces from the intended design with an inflation increase will vary by vehicle and inflation pressure, but generally results in less than optimal performance for the tire application on the vehicle."
"
Now we all know that if you run the 2nd generation Prius at the door placard pressures these contact-footprint forces will be all screwed up. Because we see that excessive wear on the outer edges of the tire. If you run 42/40 you see much more even wear, and that means the contact-footprint forces are much more even across the tire width.

We also know of the situation with the Ford Exporers, where the placard pressure was so low the sidwalls overheated at highway speeds in desert areas. And people died.

So, my comment to the general public is a bit Reaganeque - "Trust but Verify". Because that verification may save your life. I find this article, not mentioning at least the Eplorer part of recent history regarding tire pressures to be disenguous.

I'd also point out that there's a big difference between adding a couple of pounds over the placard--which on a 2010 means going from 35/33 to 38/36--and inflating to the max sidewall pressure, which is (more or less) the tire manufacturer's absolute upper limit for the tire, and would almost surely result in decreased handling/braking, and increased wear in the tread center, along with the very real possibility of catastrophic failure. I've *always*, regardless of what I'm driving, kept the pressure a couple of pounds over the placard recommendations. Never had a wear, performance or safety problem, and I do see MPG gains. It is, as other posters mentioned, a possible comfort issue. The Prius (maybe not the V, dunno about that, haven't driven it) is so softly sprung that for me, the comfort is a non-issue. Just my $0.02.
Regards,
Tim

Hi,

It is a shame the article dates from Oct. 11, 2008. By now, the reporter has moved on and follow-up comments are lost. But this is an important area, especially on the race track where optimum handling can be the difference between a money prize and another 'day at the track.'

• Overall guidelines - an excellent check list but
• Trace pressure answer -
• Fluke temperature procedure -

Notice the pattern? Race teams are using the tire temperature to set the pressure for optimum contact and wear. This in turn ensures optimum handling and avoids a time wasting tire change. So this is what I did with a $40, non-contact, IR temperature gauge after doing some literature research:

Important charts:


These charts show there is a point of diminishing returns. Max sidewall tire press is not necessary and over max sidewall is unlikely to do any better.

Field Temperature Measurements:

I found that higher pressure both reduced the tire heat load and equalized the temperatures across all treads.


Not Mentioned, Alignment:

This is an area I remain concerned about yet the article made no mention of it. The problem is fixing the alignment after the tire wear is evident only helps the next set of tires. I already have a set of alignment numbers and will fix mine at the 5,000 mile maintenance trip.

Bob Wilson

My experience is similar to yours Tony. I share my experiences but am careful not to make a recommendation. I run 44+++ and have very even tire wear. I enjoy the firm ride and feel more connected to the pavement. I expect 80k out of my current set of integrity's

If wear in the tread center is a function of inflation pressure, then it is strong evidence that the door placard pressures on my most recent non-hybrids (Subaru, Honda, Ford) are too low.

The article also quotes: "On wet surfaces, lower inflation pressure generally produces slightly higher braking traction – until the onset of hydroplaning." I wish it had expanded on this, because I found the 26 psi recommended on my Honda led to unusually low onset of hydroplaning, a very strong consideration in my climate. Other sources indicate that hydroplaning speed is a square-root function of pressure.

Agreed, I've had similar experience. Re. excessive center tread wear, I'm talking about big-time overinflation, not just exceeding the placard numbers by a few psi.

Pat's right ... temper these articles with 2 pieces of knowledge:

1) The knowledge that tire manufacturers know what side of the bread their butter is on
&
2) The knowledge that many of us on PC have driven for YEARS, running higher pressures ... not only seeing higher mpg's ... but no handling issues. You WILL feel more road imperfections, but for many, that's a small price to pay.

.

Good, I like advice given that has both the pros and cons, I tend to be distrustful when people say something is either just red or green, when it is usual a mixture of the two, probably brown.

Bob,

Is the first rolling resistant graph for cold psi?

Your field test graph is that for hot or cold psi?
What does the x-axis represent and why does it have a gap in the middle?
Why is there more delta temp variation in the right-hand set of values?

Rrue of the tyre manufacturer in selling more tyres, but how does the car manufacturer benefit from increased tyre sales? It's the car manufacturer that designs the car and I assume calculates and prints the information on the placard. A nice arrangement if the car maker recommends lower psi for passenger comfort and the tyre maker benefits from premature wear...

The first charts were found in obscure SAE papers located online. I suspect they are cold pressure.

In my test, I started with cold psi and then in the field, simply let out air to get to the lower pressures.

The Y-axis is in degree F in the first chart. The gap in the middle is because I recorded the temperatures of the front tires and put a gap in the middle. Yes, it took a lot a tests. This chart convienced me to pay more attention to alignment.

Until this set of test, I was just working with tire air pressure. But when I noticed the differences in tread depth versus the measured alignment, I realized there was another area that needed attention.

The point I'm trying to share is racers are working at the limits of their vehicles and have a vested interest in tire life (as short as it is) and achieving peak performance. They live and die by temperature readings and tuning the alignment and suspension. Compared to a manufacturer saying "just go by the door jam numbers, we know best" versus those who have 'skin in the game,' I'll lean towards those who live and die by the track. <grins>

Bob Wilson

Yesterday, I increased the pressure in the tires of my new Prius from 32psi all around (where the dealer had set it) to 42f/40r and took her out for a 30-mile test spin which gave encouraging MPG results (about 2 MPG higher so far). But before settling in with the harder tires, I decided to spend some time on PC and the web to see what people (and especially, the authorities) had to say.

I quickly found that the conventional wisdom is that over-inflation is bad (unless the payload is large – then it is acceptable). For example, you can check this discussion from Car Talk:

http://cars.cartalk.com/content/advice/tirepressure.html

Here are the major issues that I am aware of and my thoughts about their positive or negative effects. I would like to collect all the factors that relate to tire pressure.

POSITIVE:

Fuel Economy - The general belief is that harder tires give better MPG because tire-wall flexing creates heat and dissipates energy. Some people claim they do not see any improvement with higher tire pressure. I haven’t seen any MPG vs. Pressure curves. Does anyone have a source?

NEGATIVE:

Ride – Harder tires produce a somewhat bumpier and less comfortable ride. More road noise is also transmitted. Again, some people do not notice a big difference.

UNCERTAIN:

Stopping Distance: Some authorities indicate a slight degradation, as in this reference from Michelin (thanks to Timberwolf), based on simulations:

http://www.wheels.ca/article/415271

However, other authorities indicate little effect or actually an improvement, as in the Goodyear and NHSTA reference, based on road tests:

http://www.nhtsa.dot.gov/Cars/rules/rulings/TirePresFinal/FEA/TPMS3.html

However, as a worst-case scenario, let’s forget about Goodyear and NHSTA, and accept the “up to 6%†degradation from Michelin as meaning that every emergency stop will be 6% longer when 35psi goes to 44psi. If we keep the average tire pressure at 41psi (42f/40r) and scale the 6%, we get an increase in stopping distance of just 4% (assuming linear change; it may actually be sub-linear).

Also, we must remember that most Prius (and other hybrid) drivers who care enough about MPG to raise their tire pressure are also going to drive a little bit slower, and this substantially reduces the stopping distance, which varies as the square of the vehicle speed. For example, I always drove at least 5 mph above the speed limit in all of my previous cars, but now I always drive at the speed limit. How much does this reduce my stopping distance for, say, a 65 mph speed limit? Answer: 15%. So my stopping distance in Prius with over-pressure tires is far shorter than what it would have been with the driving behavior I had for decades.

This brings up a parallel question: How much would the average Prius driver with 42f/40r tires have to slow down (from your normal speed) in order to cancel the effects of tire pressure on stopping distance? Answer: ONE (1) mile per hour!

Steering and Handling: Here again, the experts say that there is a reduction in performance, but assuming it is also 4% and that one is driving 1-2 mph slower, shouldn’t things even out here also? If anyone has quantitative data or references about this, please let us all know.

Wheel bounce: This is mentioned in the Car Talk article. Of course, a harder tire is going to bounce more on bad roads. This can cause the contact patch to oscillate more in size and even go to zero (when the tire actually leaves the road!). Although this topic could be moved to the negative category above, I decided to leave it here with a common-sense warning. If you are driving fast with over-pressure tires (or even with rated pressure tires) on a bumpy road, SLOW DOWN! This should usually make up for the tire stiffness and also give you more time to see/avoid road hazards.

Tire Explosion: For an undamaged tire, there should be no issue if one stays below the max. pressure rating on the tire’s sidewall. For a damaged tire, this could be more of an issue, and people running over-pressure tires should be careful to have professionals examine any tire that has suffered a puncture or impact of any kind or that appears damaged in any way. However, a tire exploding at 35 psi or at 42 psi is going to be a serious problem either way. Since higher pressure tires flex less in the sidewall, shoulder and tread areas, the fatigue-induced failure rate (sidewall lifetime) should be lower.

Tread wear: Although one might think that higher pressure would increase tread wear from increased pressure on the road near the center of the contact patch, Prius users with high mileage on over-pressure radial bias tires usually report no such effect. In fact, they say the major wear is still at the two outer edges of the contact patch (near the shoulders), just as it is at rated pressures. The foregoing relates to differential wear. As to total wear rate, I can see that a smaller contact patch would wear faster because the wear is distributed over a smaller area. This comes down to gas savings vs. increased tire cost, but I haven’t seen a dollar comparison.

SUMMARY:

There are people out there with a lot more knowledge about automobile tires than I have, and if they say it is a bad idea to drive on over-pressure tires, we need to consider that seriously. However, because of potential liability issues, no professional or tire manufacturer (nor the writer of this post) is going to recommend that you to drive on over-pressure tires. Can you imagine someone driving with over-pressure tires and having an accident (which may or may not have been caused by tire pressure) saying: “Michelin said I could do it, and my car bounced (off a deer??) causing me to skid into a bridge abutment which has led to my paraplegia. This is their fault.â€

I would like to end by asking two questions. What factors did I leave out? Are any of my concepts above wrong, and if so, why? Any comments or opinions would be appreciated.

Thanks and sorry for the long ramble.

- g

Same results here. I'm running 44psi (cold). MPG is 5+ better than when I ran with 34psi. I see no changes in handling, but the ride is definitely harsher. Tire wear is even, and seems to be be excellent. I expect to get 50M+ on my original tires.

See the tread on the pic ??



That's what kind of 'irregular wear' I get from running my tires at 48lbs, front & rear. These Michelin Hydroedges have over 70K on 'em. The pic was taken about 9k miles ago but no point in taking a new shot ... they still look the same ... meaning any wear is un-noticable. Not that I'm suggesting what anyone else should do ...

On the other hand, after running the skummy GoodYear Integrities at 'recommended' pressure ... boom ... they were shot in under 12K miles.

same here.
al my cars for over 10 years now ( work and private ) a total of 7 cars/vans/mpv always overinflated and never any trouble.