What do you keep your psi at?

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Brogy

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Just curious, pretty straightforward question.

I usually kept mine at 32 (recommended on the door jamb), but I increased to 35 this last week, and noticed an increase in MPG. So im thinking of bumping it up to 38 and see what happens.

Does anyone keep more PSI in their drivewheel tires, and do you notice a difference?

But then again I do a lot of highway driving, I keep a safe distance so fuel economy is a priority over stopping distance and traction.

Also whats your method? I generally check once a month with a cheap pencil gauge, I figure it's gotta be within 2 psi each way, and 2 psi isnt worth spending $20 on something that'll be 1 psi more accurate.

I also find my tires do not lose air that much at all, maybe .5 psi every 3 months.

Scary story too, about how much some people care , when my sister got her car, it had 28 psi in one tire, 60 psi in another tire, 40 psi in another tire, and 15 psi in the last tire. Recommended was 32.

darthmaul.gif
 
I keep mine at 26 PSI.

The only tires that my size came in were "Extra Load," which results in this weird pressure setting for my 3,400-lb truck.

You can keep raising your pressures to gain slightly better mileage, but you're trading tire life. Pressures that are too high will wear out the center of the tread before the sides. A guideline I've used without problems is the following - 88 lbs of load rating for every 3 lbs of pressure. So I take the load rating of the OEM tires, use the guideline to find out what load they're taking at the OEM-recommended pressure+2, then convert that to what my new tire's load rating is.
 
I am not sure what bdai is trying to say, but if I read between the lines I think his methodology is incorrect.

First, tire loading is linearly related to pressure. It is a curve. And even though it is a slight curve (and you might develop a "best fit" that is linear, that line would not go through the origin.

For example, a P205/65R15 has a maximum load carrying capacity of 1400# at 35 psi. One might INCORRECTLY conclude that the tire can carry 40 # for each psi. However, the load carrying capacity at 26 psi, is not 1040#, but 1113 # as per the tire load table published by The Tire and Rim Association - the US tire standardizing body.

Another tidbit is that the shape of the tread of belted tires - and radials are always belted - is relatively insensitive to changes in pressure. The wear pattern doesn't change much with inflation pressure.

Another confusing factor is that driven tires (as opposed to freewheeling tires) tend to wear in the center of the tread, where steering tires tend to wear in the shoulders. Most people assume that because their RWD wears the rear tires in the center that this must be due to the loading, however, the rears of FWD tend to wear fairly evenly , but at a much slower rate. Further, the front tires of a FWD also tend to wear fairly evenly, because those front tires both steer AND drive and not so much because of the loading. Please note that alignment plays such a large role in even wear, that it is difficult to sort out these effects unless the alignment is closely controlled and the actual driving conditions are identical when comparisons are made.

Vehicle engineers spend a lot of time in the process of selecting the tire size and inflation pressure and are required by law to put both of those on each vehicle that is sold. It is a reasonably observed that ride is a major consideration in this process - and it is also reasonably observed that dry traction is not really an issue - so adding a little more pressure would result in some improvements in fuel economy, wet traction, etc. with only minor sacrifices in other areas.

Regarding Extra Load tires. The load curve for an extra load tire is merely an extention of the load curve for a Standard Load tire. So if you happen to get Extra Load tires, the load carrying capacity is the same at the same pressure as a Standard Load Tire - it's just that the XL uses more pressure to get the higher load carrying capacity.

BTW, the maximum pressure listed on the sidewall of a tire is rarely corresponds to the pressure where the maximum load peaks. Typically, the maximum load occurs at a lower pressure.

The exceptions are LT metric tires, and - for some odd reason - S and T rated tires made by the Michelin group (Michelin, Uniroyal and Goodrich) Every thing else tends to show a higher maximum pressure than the rated load pressure.
 
After much experimenting I run mine @ 35 front & 33 rear. Both 3psi above recommended on an '04 Accord. I've no idea about MPG but did it for better handling.
 
I run 40 front, 30 rear. Door sticker is 30 F&R.

I found this handy dandy Formula for calculating new PSI for tires with different specs from stock.
OT=Old Tire
NT=New tire
DS=Door sticker

OT max. load X [(DS psi./OT max psi.) X (NT max psi/NT max load)]= new tire pressures...
example: 1609 X [(32/35) X (44/1709)]=
1609 X [(.9143) X (.0257)]=
1609 X .0235 = 37.8 psi Front...

1609 X [(35/35) X (44/1709)]=
1609 X [(1) X (.0257)]=
1609 X .0257 = 41.3 psi Rear
It told me 30psi for the F150 with oversize goodyear fortura silentarmor. Looks good.
smile.gif
 
I set my PSI based on the lowest expected outside temperature. If the lowest expected temp during the month is say, 20F, and my truck calls for 29PSI cold, I will set mine at 30 PSI for 20F. This means that at 50F, my tires will be +4 PSI over placard. I then adjust as necessary as the minimum temps increase. It may sound like a lot of work but it's nada once you get used to doing it. This way, my tires will "never" be underinflated due to temp fluctuations.
 
Mine were chalked for going around corners. 36/28. However a little nervous out back so increased to 32. Door placard is 32/32
 
Quote:

I am not sure what bdai is trying to say, but if I read between the lines I think his methodology is incorrect.

First, tire loading is linearly related to pressure. It is a curve. And even though it is a slight curve (and you might develop a "best fit" that is linear, that line would not go through the origin.

For example, a P205/65R15 has a maximum load carrying capacity of 1400# at 35 psi. One might INCORRECTLY conclude that the tire can carry 40 # for each psi. However, the load carrying capacity at 26 psi, is not 1040#, but 1113 # as per the tire load table published by The Tire and Rim Association - the US tire standardizing body.

Another tidbit is that the shape of the tread of belted tires - and radials are always belted - is relatively insensitive to changes in pressure. The wear pattern doesn't change much with inflation pressure.

Another confusing factor is that driven tires (as opposed to freewheeling tires) tend to wear in the center of the tread, where steering tires tend to wear in the shoulders. Most people assume that because their RWD wears the rear tires in the center that this must be due to the loading, however, the rears of FWD tend to wear fairly evenly , but at a much slower rate. Further, the front tires of a FWD also tend to wear fairly evenly, because those front tires both steer AND drive and not so much because of the loading. Please note that alignment plays such a large role in even wear, that it is difficult to sort out these effects unless the alignment is closely controlled and the actual driving conditions are identical when comparisons are made.

Vehicle engineers spend a lot of time in the process of selecting the tire size and inflation pressure and are required by law to put both of those on each vehicle that is sold. It is a reasonably observed that ride is a major consideration in this process - and it is also reasonably observed that dry traction is not really an issue - so adding a little more pressure would result in some improvements in fuel economy, wet traction, etc. with only minor sacrifices in other areas.

Regarding Extra Load tires. The load curve for an extra load tire is merely an extention of the load curve for a Standard Load tire. So if you happen to get Extra Load tires, the load carrying capacity is the same at the same pressure as a Standard Load Tire - it's just that the XL uses more pressure to get the higher load carrying capacity.

BTW, the maximum pressure listed on the sidewall of a tire is rarely corresponds to the pressure where the maximum load peaks. Typically, the maximum load occurs at a lower pressure.

The exceptions are LT metric tires, and - for some odd reason - S and T rated tires made by the Michelin group (Michelin, Uniroyal and Goodrich) Every thing else tends to show a higher maximum pressure than the rated load pressure.

Click to expand...
I think you've misunderstood my 88 lbs per 3 PSI thing. You divided 1400 lbs load rating by 35 PSI, which isn't what I meant. "Correctly" (still under debate, I guess
smile.gif
) using my algorithm, to get a 1113 lb load rating with that size tire, you would need [(1400 - 1113) / 88] * 3 = 9.78 lbs reduction in pressure, giving you roughly 25 PSI. Pretty close. Of course, if you're going from a 28" tire all the way up to a 37" mudder, this formula goes out the window. I understand that.

Regarding what you said about the shape of the tread changing with pressure, I would tend to agree when it comes to summer tires. I can get a full tire footprint whether I'm running 20 PSI or 44 PSI on my A/Ts, but when I ran 32 PSI on my winter tires (the XL ones), I lost about an inch of contact on either side. Adjusting to 26 PSI gave me a much better footprint. However, even with summer tires, wouldn't you expect an increased load near the center of the belt if pressures were too high? I mean, if I inflated my A/Ts to 44 PSI, I would still get a full contact patch, but the load would not be evenly spread. The center tread blocks would be more "compressed" than the outer ones.
 
Coot uses the "I before e except after c" formula then follows the recommendation on the sticker adhering to the vehicle.

Currently, 35 psi recommended so that's the inflation pressure used.

The grass grows and the rivers run so it must be an effective pressure.
 
At my shop we usually air up 50 sets of customer's tires weekly. unless they are D or E loadrating on 3/4 ton or heavier, they ALL get 35psi. NO ONE complains because that would mean they would have to air them up themself, wich most consider troublesome.

Have noticed at least one of every set of 4 will smell strongly of dog urine. Any body got a formula to compute why sometimes all four smell that way?
spankme.gif


Perhaps getting a life would quell the need for some formulas and computations?
hornets_nest.gif


Bob
 
The owners manual for both of our vehicles calls for a an air pressure of 30 pounds before the vehicles are driven in the morning. We run 33 pounds in both of them.

I have an air compressor digital readout pressure checker.

When we are going on a long freeway trip, run 34 pounds cold air pressure in them.

Before I had an air compressor would stop at a station and pump the tires up to about 3 - 4 pounds higher than I wanted in them then would check them in the morning and adjust if necessary before the vehicle was driven.

Check them once a month.

Pressures need to be checked while the tires are cold in the mornings before the vehicle is driven!

Last two vehicles we had called for 35 pounds in the owners manual and we ran 38 pounds of cold air pressure.

Many vehicles have a sticker on the drivers door or under the hood telling you the recommended air pressure.

Don't go over the cold max safe pressures indicated on the tires. That will vary with different brands - types of tires.

With high gas prices which will be even higher when summer driving season kicks off, proper tire pressure, rotation, etc, can help gas mileage too!

Buy your own digital tire gauge. Don't trust the ones at the station!
 
I use nitrogen which keeps the temp down a bit so I go with what it says on the door label.
I like a softer ride.
 
38 F&R as per gas filler flap on my '04 F-150
32 F&R as per door jamb on my '02 Protege5

I built my own filler with an industrial pressure gauge in .5 lb increments.
 
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