I posted this on edmunds for educational purposes of young drivers (you older folks already have your minds made up)

Manufacturers design the tires to match up with the cars. They expect a specific tire surface interface area on the pavement to create a desired safe handling characteristics.

Just because a tire has a maximum pressure rating does not mean it should be achieved. If you put the same tire on a 3500# car and a 4400# car you'll want more pressure in the heavier car to create the right surface area. Increasing the pressure on a lighter car however lessens the tire contact area and thus create a longer braking distance as well as less surface area for grip.

Granted there is some "play" in these numbers but increasing pressures 30% over design is pushing the safety and handling aspect of the tire design for good FE.

Tires are designed with a maximum pressure so that they can achieve a wide range of applications, not so you can pump them up hard on your TCH.

Also (someone mentioned the max on the Michellin was 44 psig) running at 42 cold pressure on concrete highways at high speed in hot weather will probably exceed your maximum tire pressures. If you blow a tire and wreck your car don't put it past your insurance or whoever you decide to sue to check your pressures on your other tires or for that matter your comments on this website to see if you purposely exceeded recommended pressures. (I'm not a lawyer, just spend a lot of time tring to figure them out at times)

I realize this won't change the minds of you die hards. Don't even want to. However many novices watch these forums for ideas and "buy into" just about everything they read from synthetic oil, high tire pressures and running in neutral. All these things should be (re)considered after research and study, not just as a "I wanna do it to" attitude.

 

Agreed. Each person should do their own testing. Each person should do their own research. With that said, however:

I had this "tire pressure" discussion with the World's Greatest Hypermiler (we all know him) and he convinced me personally (after I had similar concerns about safety/grip/wear etc) that higher tire pressures are safe and that they do not increase untoward tire wear and that they DO help MPG.

I ran my 2004 HCH for about 20,000 miles with PSI about 20 higher than recommended. The car rode and handled fine, and the tires wore normally. I'm running my TCH at 46-50, and I might go higher.

I have the advantage of living in DRY Phoenix and grip in the rain is not an issue. My driving style does not lend itself to the tires losing grip under any circumstances.

On Synthetic oil, well, there is really no solid, undeniable, scientific evidence that it helps your MPG. What it DOES DO, without a scientific doubt, is protect your engine better than Dino oil. So if you want to protect your engine in the best (not cheapest) way, synthetic oil is better. Not necessarily for increased MPG, though. Synthetic oil in my 2004 HCH made ZERO difference at all in MPG, and I checked it carefully.

To each their own, and Your Mileage May Vary (but at least you can hope it varies UPWARD !!!)

Lars

 

In any post of this type, would everyone please reference a reliable source for any knowledge-based claim? There has been a lot of surmising and hypothesizing in the various hybrid forums about both the PROS and CONS of tire inflation, but I've tried to trace much of it to a reliable source, and can't. (Check for lively discussions of tire pressures and risks in any Insight or Prius forum, where the born-again hypermilers can be found.)

With NO intended reference to the original poster, some of what has been said about performance risks (handling, braking, tire wear, etc.) is simply sensible-sounding hearsay, or based on exaggerated examples. Let's all educate ourselves (me, especially) with FACTS about tire pressures and performance.

The spirit of WVGasGuy post is quite valid, and that is to use judgment and do fact-based research on the benefits and potential risks before filling your tires.

 

Besides common sense, a degree in civil engineering (highways emphasis) and reading car magazines for 40 years I offer these quick references.


From NADA
http://www.nada.org/PrinterTemplate....ontentID=14267

Check your tire pressure at least once per month, and especially before a long trip. Remember underinflation is a tire’s number one enemy, because it can cause damage that may lead to tire failure. However, overinflation can cause uneven wear plus handling and stopping problems. Use the manufacturer’s recommended air pressure listed on the sticker of your vehicle’s door jamb or owner’s manual as a guide. Always check the pressure of your tires when they are cool or cold. Driving heats up tires and make the reading incorrect.

From GM:
http://www.gm.com/company/gmability/...ou/manual.html

If your tires have too much air (overinflation), you can get the following:
Unusual wear
Bad handling
Rough ride



Car and Driver
http://www.caranddriver.com/dailyaut...fety-tips.html

However, overinflation can cause uneven wear plus handling and stopping problems.

 

With that said, the recommendations (references not shown) if you load a car you need to add pressure. This pressure helps maintain the contact patch to the road. The load would "flatten" out the tires and increase the contact patch possibly causing overheating (the same problem with underinflation). Still you should not exceed the limit of the tire (read the sidewall). Extended highspeed driving requires a higher pressure of a couple of pounds to reduce heat. And your pressure will change 1-2 pounds for every increase or decrease of temperature.

As for auto crossing and Nascar, etc they constantly play with suspensions and pressure settings to max out speed. They also change tires often or don't worry much about long life. Drag racers decrease pressure for traction but they don't have to worry about going around curves. They also bolt the tires to the wheels so they don't slip off while spinning. There are a lot of things going on in the race world that don't apply to everyday driving

 

I didn't start this thread to try and tell experienced drivers what they should or shouldn't do. It's simply a warning to inexperienced (if anyone will admit to that) drivers to take caution about trying everything they read.

For example lars-ss talks about running tires at 50 psig. Just realize when you read his posts that he lives in Arizona and is probably not driving on hilly interstate roads in the rain at 75 mph. I wouldn't do that with 50 psig in my tires in West Virginia.

 

I would recommend at least 35psi as that is Toyota's recommendation for highway usage.

Running higher pressures (staying below the max rating) can affect ride, noise, tire wear and handling. However, pressures higher than the car manufaturer's recommendation are not always a problem. Sometimes ride softness/comfort gets the nudge over other factors - witness the orginal 24psi recommendation by Ford for the Explorer and the Firestone tire issue.

I suggest reading Driving Under Pressure for a glimpse at how higher pressures don't always cause poor handling and bad tire wear. If that link stops working, here is another link to the same article posted on GH

Research issues and keep an open mind

 

I wonder if the savings in increased mileage by running high tire pressure is somehow offset by the cost of more front end component repairs down the road?
Robbing from Peter to pay Paul sort of thing?
J

 

Well I don;t know of any scientific report, but I know that I had 1730 miles at my foirst oil change, my tank average was climbing slowly and was at 36MPG when I did first oil change and put in 0W20 Synthetic. I then went and filled the tank. I have 70 miles on this tank right now and the tank average is about 40MPG. I attribute some of this to 0W20 Synthetic and some to more engine break in and me learning to drive the car better, but a 3-4 MPG boost over night, some has to go to the oil. I can feel the car running smoother on cold startups. I was sold back in 91 when I tried it on my wife's Celica an noticed the same thing. At that time I went from 10W30 to Synthetic 10W30 so no weight change, but noticed the smoother running engine immediately. It was most evident on the initial cold start as the engine just purred right away instead of that normal minor struggle when first spinning up. When I moved to V6s I still used Synthetic, but the fact that V6s run smoother and had more power anyway I was never able to really feel the difference in them, but assume I gained a minor improvement in MPG anyway.

 

Yep _ I forgot to mention that one area synthetics shine is at startup. The thinner oil and uniform-size molecules "cling to" the component parts better when the car is not running so that less of the oil drains away down into the oil pan and more oil is on your engine parts at startup.

And the reason some people swear about MPG gain is the decreased friction that the thinner, "slipperier" synthetic gives you.