Tire Pressure questions

[SteveSRT8]

Originally Posted By: Garak
I don't stray far from the placard recommendations. Your mention of someone managing vans and trucks is telling. Vans and trucks in 3/4 ton size and up tend to call for a higher pressure setting anyhow.
What are you gaining? You're not gaining significant fuel economy. You're not improving the life of your tires. You're not improving ride. You're not appreciably improving handling. What's the point?


Our 3500 service vans call for 55 front and 80 rear cold inflation. The rear tires are already at max pressure cold. One of the HUGE reasons for the stagger is handling. If you are a bad driver and whip the truck from lane to lane the tail could wag if the rear tires are low. VERY dangerous. There are very real liability issues if the truck is in an accident and anyone starts looking closely at the vehicle.

Forget fuel economy in a 9200 pound truck. There isn't any!

I can see isolated instances where more pressure could help you if you experience the edges wearing or other things, but otherwise...

 

[troberts]

On the other end of the spctrum....
People on the Prius forums claim all sorts of mileage benefits from runnng way higher tire pressures. I haven't experienced the same. My Prius commute includes twice daily crossings of the Fred Hartman suspension bridge over the Houston ship channel. The wind off the bay, normally 15-25mph, is at right angles to the directions of travel. Include mixing it up with the container trucks, chemical and gasoline tankers, etc., and you see what kind of air turbulence I deal with. Bringing the cold inflation pressure up from the recommended 35-33 to 38 all around definitely decreases the "squirminess" factor as I white knuckle along. Can't testify to any mileage increase but I do notice the interior component vibrations increase with just that little change from Toyota's recommendations. Wear is dead flat across the width of the tread. This adjustment only applies to the brand of tires I run currently and is different from what I had to run wih the OEM set.

 

[Garak]

Originally Posted By: SteveSRT8
Our 3500 service vans call for 55 front and 80 rear cold inflation. The rear tires are already at max pressure cold.

Quite right. And such vehicles (and even larger ones) are such a pleasure to drive when unloaded, too.

Back in around 1995, when my LTD was totaled off, I was relegated to an orange GMC one ton yard truck with ridiculously low gearing as my daily driver. With absolutely nothing to haul, the experience got mighty old in a hurry!

 

[CapriRacer]

A couple of thoughts:

First is that talking about tire inflation pressure without any other reference isn't the whole story.

Passenger car tires are typically in the 26 to 35 psi range. LT tires are in the 50 to 80 psi range. Truck tires up to 120 psi. Some aircraft over 300. Bicycle tires have pressure ranges from 30 psi to 130 psi (OK, bikes is not my area of expertise and I had to look this up. I don't claim that this is anyway accurate for bikes.)

If we confine our discussion to passenger car tires, the load carrying capacity varies by inflation pressure and tire size. Put another way, if a vehicle manufacturer specifies a small tire, he would also specify a high inflation pressure, while if he specified a large tire, he could specify a low inflation pressure. There are tradeoffs for each, and there are ways to compensating for those tradeoffs in the vehicle suspension.

So for example, if someone says he is using 40 psi - well, relative to 35 psi, that's not a large difference. But relative to 26 psi, that's a HUGE!!! difference.

Yes, higher inflation pressure do result in lower rolling resistance (and better fuel economy). This is easy to prove.

Where the problem lies in that I know of no one who has done the rest of the testing - traction, handling, treadwear, ride, durability, etc. What we are left with is anecdotes which are notoriously unreliable. I even had someone tell me there was no evidence whatsoever that there were ANY issues about using tires at the max pressure listed on the sidewall. A simple google search would have revealed this to be a falsehood.

Without knowing what affects higher pressures have, there's a risk - and one worth being serious about as tire failures have been known to result in fatalities.

So I urge caution. Be careful about using higher pressures. My personal advice is to use the pressure listed on the vehicle tire placard (if you are using the same tire size listed there). I personally use a few psi more, but I am using older cars where the tire sizes were a little small.

 

[Garak]

Originally Posted By: CapriRacer
Where the problem lies in that I know of no one who has done the rest of the testing - traction, handling, treadwear, ride, durability, etc.

Exactly. The automaker has done a lot more testing than I certainly can do. And of course, we don't need to panic about a couple pounds in either direction. But, I don't like to second guess their directions.

As for bike tires, my bicycle is supposed to be at 80 psi, I believe. Fortunately, I have a gauge that will go that high. Incidentally, bicycle tires are the only ones I've ever seen that have inflation pressure instructions right on them (rather than a max). My bicycle tire says inflate to 80 psi or some similar wording. I suppose bicycles don't have a lot of room for placards or a glove box for an owner's manual.

On the other hand, my buddy has a big riding mower at his farm. One of the deck tires need to be replaced and he didn't trust the repair person to inflate it properly. I told him to check the manual. He wanted to check the sidewall. The sidewall listed a maximum pressure of 60 psi or something similarly high. The manual called for 6 psi. Of course, it's not holding a lot of load and it's not involved in handling or high speeds. But, that is a significant difference.

 

[faramir9]

It looks as though recommended inflation pressures have gone up in the last few model years: http://www.firestonecompleteautocare.com/tires/tirepressure.jsp

Not much, but enough to notice. See the latest generation of all the standard mid-sizers, etc. Is this part of the CAFE wars? Also may be along the same lines as many tires now stamped with 51 psi sidewall max, instead of 44 psi. That makes me chuckle, when I see XL tires maxed at 50.

It seems clear to me that most OE recommended pressures are partly comfort-driven. My Saturn specs 30/26; I run 34-35 front and 32 rear. Definitely handles better, no uneven wear and no noticeable decrease in traction. Might be tire-specific, too. The H727 is a great tire overall for normal daily driving, but the sidewall could be a hair stiffer....

 

[Garak]

Originally Posted By: faramir9
Not much, but enough to notice. See the latest generation of all the standard mid-sizers, etc. Is this part of the CAFE wars? Also may be along the same lines as many tires now stamped with 51 psi sidewall max, instead of 44 psi. That makes me chuckle, when I see XL tires maxed at 50.

Well, a little fuel economy bonus couldn't hurt, at least from the manufacturers' standpoints. I think pressures increased a bit after the Firestone/Explorer business, and comfort has always been an issue. Heck, as I mentioned, the Audi manual had a loaded/high speed pressure and a "regular" pressure. The ride was noticeably different.

The point is, a few pounds up isn't a big deal. It's certainly not the same thing, though, as inflating to maximum sidewall pressure.

 

[Ken2]

Bicycle tire pressure calculator
http://www.biketinker.com/2010/bike-resources/optimal-tire-pressure-for-bicycles/

Bicycle tire pressure Android app!
https://play.google.com/store/apps/detai...mVwcmVzc3VyZSJd


Motorcycle riders get into these same discussions. There is a recommended inflation pressure sticker on the swingarm of my Suzuki V-Strom. It shows 33 psi front and 36 rear--39 for riding with a passenger. The sticker on the bike frame that shows max GVWR gives the tire pressures required at max, 42, and many think this is the required pressure all the time. The tire sidewalls also have their max, usually 42, and other riders think this is the tire maker's "right" pressure.

One thing to keep in mind is that tires need to be normally warm for best traction. This warmth is generated from flexing as they roll, and perhaps also from pavement temperature. Motorcycle race tires even get electric tire warmers to get them ready to race. So--a couple of psi less in cold weather and a couple of psi more in very hot weather will help even out the tire temperature and the traction--really, really important on two wheels. Tire wear is a big deal for motorcycles, a coupl'a hundred dollars for a rear that gets several thousand miles of wear (not ten thousand miles) means that a too-warm tire wears even faster than necessary. Riders to Alaska often arrange for a tire replacement along the way.