Tire over inflation

In my 2005 I ran very elevated pressures (60psi) for most of its miles. The struts wore out to the point of noticeable somewhere around 130k. Everything else seemed fine and it now has 165,000 on it. Considering OE struts tend to show degradation by 60k I don't think the higher tire pressure did anything negative except increase rattles.

 

Just a comment on semantics -- to me, tires inflated to the sidewall maximum are 'fully inflated', not 'over inflated'.

Perhaps this relates to past examples of car manufacturer recommendations that proved to be under inflated -- my '86 Accord that hydroplaned too easily, Ford Explorers that rolled over with defective Firestone tires exacerbated by a too-low Ford pressure recommendation and several other factors, and Gen2 Prii that displayed excessive shoulder wear when inflated only to the car's placard recommendation.

 

Plenty has been written about this; in general, over-inflating does not help your mileage much if any and introduces a lot of other problems such as compromised handling, harsher ride, etc. Go with the manufacturer's recommendations and check pressures often.

 

prev93 I must disagree with your post. I think plenty has been written on Priuschat about the benefits of inflating tires above the door placard values, while staying below sidewall max and maintaining the front/back stagger. Increased mileage and improved handling have been reported, but of course what is considered an improvement varies from person to person. If improved mileage is someone's key measure, then bumping up the pressures is probably worth it and the increased harshness of ride would be an acceptable tradeoff. Or for some, a 2 mgp improvement would be considered minimal while the increased "feel for the road" would be unacceptable.

I'd say bump up the pressures in 2 psi increments, while maintaining the stagger, and checking pressure of cold (1st thing in the morning) tires. See how the car feels with the bumped up pressures, and if ride/handling are OK, then bump it up again, until you reach the max sidewall value or ride/handling becomes uncomfortable.

 

Increasing inflation pressures above the recommended levels reduces the area of the contact patch. This can increase braking distances at low speeds (not hydroplaning speeds) on wet surfaces since fewer sipes are in contact with the road. Most notably for a Prius, it will exacerbate the annoying transition between regenerative and friction braking, not only because of wet slip, but also because the smaller patch is less able to conform to road irregularities.

My recommendation: If you own more than one vehicle, and one is not a Prius, then increase the tire pressure on the other vehicle -- you will save more gallons of fuel.

 

Who's recommendations? The car manufacturer or the tire manufacturer? I would argue that the Prius is safer as you get closer to the tire max sidewall pressure (a little below if max is 44psi). New tires do not deform quite as much as older tires when inflated above manufacturer recommended levels as is evidenced by tire wear. Remember we are talking about modern narrow tires. Not old school wide tires.

 

I put on 68,000 miles on Michelin Energy Saver AS tires, 195x65x15. Last month they
were approaching the wear marks and I changed them out to Ecopia EP422.

The Michelin's were approaching the wear bar indicators and all four tires were always
inflated to 44 PSI which is maximum sidewall pressure, and rotated every 7,500 miles.

The tires wore perfectly and I have no noticeable problems with the suspension system
and It appears to be just as good as when new.

 

i think that anything below the max on the tire is not over inflated.

i think running at max tire pressure (44) would have at most a nominal effect on suspensions. i think going significantly higher than 44 does not give enough back to be that beneficial. also, insurance companies look for excuses to not pay out on claims. dont make it easy for them

 

Which manufacturer's recommendations? The car, or the tire?

On previous cars with tires inflated to the car maker's labels, I've suffered premature hydroplaning and tire wear patterns indicative of underinflation. The higher pressure allowed by the tire makers has given me better results.

 

this illustrates Auto Manufacturers changing tire pressures to hide suspension defects in my mind. the #1 reason for following Car specs is ride comfort which begs the question; when getting EPA mileage ratings what the tire pressure is then?

 

32 seems really low for the Prius, i'd say 38/36 is a good compromise but mine are 44/42 and i deal with the increasing noise and harshness. I traded my dad cars for a weekend so he could take my Prius to San Fran vs. his Ram 1500 and with the tires being set at 36/34 and right after a fresh oil change he averaged 48 the entire trip from SD to SF at highway speeds of 75. The Ram 1500 says tires should be at 35/35 but max sidewall is 52/52 and at that PSI I am getting 25mpg on the highway at 65mph and the truck seems to coast a lot better vs. instantly decelerating when you let off the gas.

 

I currently have my tires at 50/45 to explore that end of the inflation zone. IIRC, max inflation is 55psi.

What I have found is:

-The ride depends on the road. If you have crappy roads you will have a crappy ride. On smooth parts the ride is nice. Bumps are a little sharper but not tremendously so.
-Fast corners are more easily taken, and higher speeds are possible with comfort.
-No rain lately, so no wet road examination. However, from past experience, I will NOT tempt fate with experimental driving on high inflation pressures.
-Wear pattern appears even across the tire. No crowning evident in the dust/dirt on the tire.
-I know they roll more easily, so should have better performance...but I can't tell with certainty what any percentage of increase it would be. It appears to be within background noise, but my driving has not been of a scientifically repeatable form lately. Traffic gets in the way, the lure of a different route beckons, and errands abound.

As far as longevity of suspension components, I wouldn't think it would matter. The rise of the suspension would be the same only quicker except for the smallest of the vibration/movement soaked up by a lower psi tire.

The jarring might affect other components, though.

 

another thing to consider; if you want something near the mileage rating of your tire, you pretty much need the pressure to be at the max listed on the tire.

 

It sure doesn't on my tires. I think in order to reduce the contact patch on modern radials, you'd have to overinflate them to the point of bursting.

REV

 

that is true and remember until you exceed the max tire pressure by the TIRE manufacturer, you are not in any danger of reducing tire contact due to over pressure. with steel belts stabilizing tire shape, i am betting you must be waaay high to bend them out of shape.

 

I always wanted to know the answer to this. This well could be one of the EPA cheats. Well known EPA cheat: 0% ethanol, 91 octanes during EPA testing.

 

Consider a freebody diagram with the pressure behind the tread equaling the pressure on the road. If the contact patch were to remain the same with increased pressure, the net vertical force on the tires would exceed the weight of the vehicle -- an impossibility!

Tirerack says: An overinflated tire is stiff and unyielding and the size of its footprint in contact with the road is reduced.

The contact patch does not get narrower with increased pressure; it gets shorter.

 

The vehicle manufacturer. You have to believe that the engineers who tune the suspension, selecting spring and shock parameters, assume "properly" inflated tires. The tire manufacturer has no in-depth knowledge of a vehicle's suspension performance and that is why they do not specify inflation pressures--- except for specifying a fairly high maximum inflation pressure to best pass speed rating tests!