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ski
03-21-2002, 11:04 PM
On my truck I have wheels that are both larger and wider than the stock ones (not hugely oversize, just a little). As a result, the vehicle manufacturer's recommendations for tire pressure are no longer valid (I assume). So how can I find the right tire pressure to run?

As discussed in this thread (http://boards.straightdope.com/sdmb/showthread.php?threadid=69842&highlight=tire+pressure) , the max pressure on the sidewall of the tire varies from that recommended by the vehicle manufacturer.

Duck Duck Goose
03-22-2002, 10:45 AM
Eh, no real clue, but I'll bump this from Page 2 for ya, let the Friday Morning group take a whack at it.

Just a guess--ask the tire manufacturer? Do they have a website?

Crafter_Man
03-22-2002, 10:59 AM
I have the same question.

I put oversized tires on our Safari minivan, because the stock tires are only made by Goodyear, and are very expensive.

Iíve been inflating them to the recommended values listed on the door jam sticker. They seem to be O.K. Ö

DougC
03-22-2002, 10:49 PM
- - - You always inflate the tires to the pressure that the tires say. The manufacturer's suggested inflation pressure is for the original-size tires, not for all tires of any size.
~
- A bigger/wider tire's inflation pressure should be lower than the pressure of a smaller/narrower tire. Depending on the vehicle, some big hillbilly off-road tires run just fine with only 20 PSI in them. - DougC

Gary T
03-22-2002, 11:26 PM
Originally posted by DougC
You always inflate the tires to the pressure that the tires say.
That would be great if the tires said what to inflate them to. They don't. They say what the MAXIMUM pressure is for that tire. They don't say what the correct pressure is for that tire on a particular vehicle carrying a particular load.

Lacking any authoritative recommended pressure, I would fill them to 10% less than the maximum.

zweisamkeit
03-23-2002, 12:50 AM
i work at a gas station, where i fill many many many many tires each day i work. the general rule of thumb is:

if the tire says:
MAX PRESS 35 psi -- inflate to 30-32 psi

MAX PRESS 44 psi -- inflate to 36-38 psi

those are the most common tire pressures, although for larger truck tires, it's going to be higher.

to be safe, especially if the max pressure listed is above 44, inflate the tires MAXIMUM to five psi below the indicated number (if it says 60, go up to 55). this, of course, doesn't apply to "donut" spare tires, which need to be at 60psi.

the reason you don't want to inflate a 35psi tire up to 35 is as you drive, the tire heats up and the air expands, increasing the pressure inside. if you already have the maximum pressure in there,well..

danvanf
03-23-2002, 01:19 AM
Proper pressure can be generally found on placards on the frame area of the drivers door. (Post or door itself) They are for a specific tire size, also listed. The markings on the tires are maximum pressure for that tire, which is where it should be only if it's at max load. (again for the tire) That becomes useful just prior to loading a pickup.

During the Firestone/Explorer mess, I heard Firestone say that the tires were run below recommended pressures, causing excessive heat. I thought, WTF, then maybe you should think about putting that in your little tire mold. In other words, to low a pressure is as bad as to high. (Not to say that some tires are designed to run at low pressures)

Now to get the proper pressure for your tire... I might have to defer, ahh what the heck, it seems it could be figured out.* We know current pressure (call it 30 psi) we can figure out tire foot print (call it 40 square inches). We know old tire width, and new tire width. (7.25 and 8) So at 8 inches wide we have 44.14 square inches of contact area. (Not taking diameter difference into account^) We were holding up 1200 lbs before, so now our pressure should be (1200/44.14) or 27.186 lbs, or about 10%
Humm, Maybe Garys got something there. :D

With bias ply tires, we'd just look and pump, radials don't work as good. The tire manufacture might know recommended road contact area, that would be helpful.


Dan

* not by me, but it seems it should be able to be.
^- or some other stuff

handy
03-23-2002, 10:38 AM
I would just call the people who make them. Why not? Im sure they would love to tell you the right answer.

scr4
03-23-2002, 10:49 AM
Originally posted by handy
I would just call the people who make them.

Wouldn't the car maker just say "don't use oversized tires"? And what do the tire manufactures know about what pressure tires are appropriate for the particular car?

waterj2
03-23-2002, 11:27 AM
And what do the tire manufactures know about what pressure tires are appropriate for the particular car?
I'd imagine they know a lot about that sort of thing. Presumably they test their tires on a variety of cars, and could at least ballpark it from the type and weight of the vehicle.

-waterj2, whose bike tires are 110psi, and laughs at all of your low pressures

scr4
03-23-2002, 11:35 AM
Originally posted by waterj2
-waterj2, whose bike tires are 110psi, and laughs at all of your low pressures

That's nothing. Racing wheelchairs use 220 psi tires.

Gary T
03-23-2002, 12:16 PM
Originally posted by zweisamkeit
the reason you don't want to inflate a 35psi tire up to 35 is as you drive, the tire heats up and the air expands, increasing the pressure inside. if you already have the maximum pressure in there,well..
Actually, that's not a concern. The specs from tire makers and car manufacturers are cold inflation pressures. Yes, the pressure will increase when the tire heats up, but they've figured that into the mix.

However, this does mean that you can't get a meaningful pressure reading from a hot tire. Check and adjust pressures when they're cold.

DougC
03-23-2002, 06:28 PM
- - - The sticker on the vehicle's door jamb assumes you are still using the OEM-sized tires. Smaller tires need higher pressures, and larger tires need lower pressures. If you put on tires that are only an inch wider the difference may not amount to much, but while regular car tires are inflated to 32-35 PSI, baby spares use 50-65 PSI and 39x15 off-road tires use 20-25 PSI.
- Some tires have a little table listing a few vehicle weights across a range and the corresponding inflation pressures, but if you only have the max load and the max-load-pressure, then you figure a percentege of the max pressure based on the vehicle's weight compared to the maximum rated load of the tire. - DougC

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