Tire over inflation

Aye but car manufacturers have an agenda to sell cars and limit liability so they choose settings that work for the masses. The average Joe wants a soft comfortable ride so tire pressures are likely chosen accordingly. It's a one size fits all approach.

True hypermilers drive slower and more cautiously than the average Joe so I believe they can do just fine with very high pressures. Like all things, there is a point of diminishing returns and high pressure or lower pressure are not perfect for all conditions as WalterLee and I often comment.

Classic A B A testing has proven that overinflated tires roll further in coast down tests. This will definitely enhance mpg, especially for hypermilers unless the road surfaces are very rough.

 

Max sidewall on my 17's is 51psi so OP has a ways to go to proper inflation, IMO. Soft tires don't have an optimum contact patch so they will hydroplane sooner and wear on the edges. Set the tp wherever you want as they are your tires. Practice some evasive maneuvers in a large empty parking lot so you won't be surprised on a public road.

 

They go for comfort and lower cost. Same reason you see big heavy duty trucks with tiny, narrow tires..cheapest tire that meets the payload and towing specs and also narrower=higher EPA MPG

 

There was a point where the overinflation of my tires made my prius a rattle machine, lowered the tire preassures to 39-37 and all was quiet again.

 

Mine has no rattles at 36K miles and 42/40 (F/R) since day 1. I can tell when they get low at around 36 or so after a cold night. I would NEVER drive my Prius at recommended pressure ... it would seem like it was plowing snow.

REV

 

I would agree, the max sidewall pressure should be fine for the tire. But it will reduce the contact patch, iirc, normally 5psi over the manufacturers, toyota's, recommendation would start to reduce the patch. If you are in wet conditions this may even help traction, but might reduce it on dry uneven roads. Underinflated tires reduce the contact patch more, and increase fuel consumption.

Absolutely, and if you like the way toyota tunes the suspension, by all means stick with the recomended pressures. But this is a personal matter, and I didn't like their tuning. I'm running 42 psi on aftermarket 205/50R17s. Try 36 psi and if you like the ride and mileage better, pump it up in increments of 2 psi utnil you no longer see improvement or reach the tire max sidewall pressure.

 

I've always wondered what the relationship is between tire pressure and contact patch on tires. Intuitively, it would seem the higher the pressure, the lower the contact patch. Of course, the important question would be in what relation do we find these items. Also, the question isn't really answered for purposes of safety if we don't also consider contact pressure.

I've found an engineer's analysis of studies done by a tire manufacturer, Avon, of contact patch pressure, and he analyzed the data to determine correlation between contact patch and pressure. Interestingly enough, the data he analyzed showed very little change in contact patch based on air pressure in the tire. While the study was looking at racing tires and ignored some aspects of sidewall deflection, this engineer clearly sees the data as "deflating" the argument that tire pressure and contact patch fit in the regularly assumed relationship of contact patch area = (weight/tire pressure).

Perhaps others can find more definitive studies coming to differing conclusions, but I thought this was rather interesting. I'd like to hear from the engineers among us about this analysis: Fact or fiction? Tire contact patch size is determined mostly by weight and tire pressure.

I think the observations collected and reported by most of us include statements that higher tire pressure produces higher fuel economy. I've experienced this, and I've figured it was due to lower tire resistance/friction and that the lower resistance/friction resulted from less contact with the road. Of course, I've seen more significant losses due to rain than lower tire pressures.

 

^^^^^^^
Under inflated tires have reduced contact patch because the center deforms. This will cause worse handling and mileage. After a certain point, though higher pressures will deform the tire enough to lift the sides of the tire enough to reduce contact patch. I don't know what this pressure is though, and is likely at least 5 psi above what is on your car's placard. The contract patch won't shrink much on lower rolling resistant tires as long as you stay under the rating on the sidewall. Then again much above 40psi you aren't going to improve fuel economy much either on any of these toyota recomended tires. Higher seed driving, more loaded cars, and wet conditions give better handling at higher pressures.

 

Where do you find this information? It seems counter-intuitive that underinflated tires have reduced contact patch, but I'm willing to accept it if studies show it to be true. The rest of it is interesting, too, but I don't know that it will change much of how I drive. I usually save the highest pressures for my highway driving, and I suppose this is appropriate given what you suggest. Do you see any safety issues associated with higher pressures (though still at max cold pressure or below)?

I know my fuel economy clearly improved when I increased the tire pressure well above 40psi. It continued to improve the higher I went, but I dropped it back due to the jaw-jarring ride. Mine is the only one I have actual knowledge of, but I've heard of others who've had similar experiences.

Also: Where does the "at least 5 psi above" the cold max pressure come from? I've not gotten anywhere close to that pressure (and don't want to try it!), but I suppose others have done it. Does this come from collected data, or is it simply part of the calculation that determines the max pressure labels to put on tires?

 

From memory, which may not be great. tire rack is always a good source
Tire Tech Information - Air Pressure - Correct, Underinflated and Overinflated

I can't tell from that if they are saying bigger patch, so you may be right. Definitely they are saying worse handling and fuel economy.

I wasn't saying 5 psi above cold max pressure, its above the pacarded pressure in your car which I think is 32 psi. Your max pressure is printed on your sidewalls, and I was recomending going over the 32 but staying at or bellow what the tire says. 5psi used to be the tolerance of car makers labeling the tires, but manufacturing changes every year. Tire rack talks about overinflated by 6psi

Hope that helps, and I take back the idea that under inflation reduces contact patch.

 

Yes, I think they are saying there will be a bigger patch with underinflated tires. One of the biggest dangers has to do with the high temps that can build up from the increased friction. I've heard tire guys say an underinflated tire is much more likely to blowout than an overinflated tire -- mainly because of the quick heat build-up (which clearly will increase pressure).

My bad. I was reading "placarded" as the notice on the side of the tire advising about the max cold pressure.

That info helps. Thanks.

I think the TireRack commentary has very little to do with a relationship between tire pressure and fuel economy (except on the low end), but it does bring up the harshness of the ride and the increased risks of damage to the tires. They've left out the warning that wheels can actually be damaged from potholes and curbs when tires are underinflated! (And don't ask me how I know.)

 

OK here is what I remembered badly. Under inflated tires have smaller contact patches in water.
Tire Tech Information - Air Pressure vs. Wet Performance

 

Exactly. This is why higher pressure and deeper sipes are the best defense to hydroplaning -- well, second to not driving at high speed over standing water.

 

The main reason for better fuel economy with higher pressure is a reduction in deformation which wastes energy.

There is a good write up on wikipedia. Scroll down to the section title "Factors that contribute in tires" after you read about hysteresis.

Rolling resistance - Wikipedia, the free encyclopedia

 

The car manufacturer's pressure recommendation is a compromise, in part to soften the ride. A few extra pounds will decrease rolling resistance, and improve mpg as a consequence. That's what I do, setting pressure about mid-way between the recommendation and max embossed on the tire sidewall.

That said, I would think any increase in tire pressures is going to increase suspension component wear. If the tire is stiffer it's going to absorb less of the road imperfections, and the suspension is going to deflect more. But an increase of a few pounds likely shouldn't make a big difference.

 

dont know the conditions of the roads in your area but around here, underinflated tires (that means to the Auto Manufacturers level) means more danger and less control when roads are not perfect.

the additional tire pressure does not compromise safety in any way and provides better control and stability on rough roads. i will trade the safety of my family over some worn suspension parts any day

 

^ If you're going to quote my posting at least read it, LOL.

"... That's what I do, setting pressure about mid-way between the recommendation and max embossed on the tire sidewall..."

 

i did and your implied concern that a pound over the maximum is somehow unsafe is groundless. for one thing, the 44 lbs is cold pressure reading. during hot summers, it is normal to see pressures 20 lbs over that and there is still no danger. there is a very large margin of safety built into that recommendation and unless you live in Phoenix which you apparently do not, you have absolutely nothing to be worried about

 

I'm just curious - what is the source for the factual statements you made in this post?

I ask because my personal experience is to the contrary on every point. I would never tell anyone else what to do. But I will report my personal experience. I inflate my tires to the sidewall limit (rear 2 lbs. less) and I have experience significantly improved tire wear and slightly improved fuel economy without any compromised handling, no harsh ride, and no observable bake performance impact. I use the sidewall pressure mostly for the improved tread wear.