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Old 06-03-2011, 02:15 PM   #1
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Tire pressure - how close is good enough?

While I absolutely love my TST tire pressure monitor system it insists on telling me when I haven't gotten all the pressures exactly equal in all the tires.

So what do other folks figure is a good enough tire pressure difference between the tires when your dealing with 6 tires? Is 1 lb difference OK but 2 or 3 too much? I figure before digital gauges folks were using the stick ones and those aren't really precise.

Also our driveway isn't level and our street isn't either so it is really hard to know if this is affecting the pressures.

Thanks!
Michelle
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Old 06-03-2011, 03:26 PM   #2
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Generally speaking I try to get the pressures within 2lbs fo each other on the same axle... so long as both are at least at or above the desired pressure. If I want 90lbs on all four rear duals, I make sure they're all at least at 90 but if one is at 92 I won't drain any air from it.

Not sceintific by any stretch but it's what I do.

Good luck...

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Old 06-03-2011, 03:45 PM   #3
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My guess is close is good - Once you start moving down the highway all the pressures will change and not in equal amounts depending on weight on each wheel and pperhaps wheel position. I'm amazed at the amount of pressure change when going down the highway on a hot day. I do try to get them close when checking and filling but don't think I've ever had them all the same. I use the pressure pro system.
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Old 06-03-2011, 03:50 PM   #4
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I have and electronic gauge that reads to psi and I get them exact, not that they'll stay that way though as just the sun on one side will raise the pressure as will road crown.
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Old 06-03-2011, 06:09 PM   #5
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Hi micd,
Use the gage of your choice to get the tires at the recommended pressure + a maximum of 5 PSI. I get mine exactly the same with a tire gage. The TPMS never reads the same as my gage. As an example, I made a chart that says 90 PSI (gage) = TPMS XX PSI reading. I did this for each tire. The TPMS may not be the same as my gage, but it is consistent in its' deviation.
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Old 06-03-2011, 06:19 PM   #6
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I get them all to 90 on my gauge. My Tire SafeGard reads 95 to 97 when I am done.
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Old 06-03-2011, 06:50 PM   #7
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Quote:
Originally Posted by GaryKD View Post
Hi micd,
Use the gauge of your choice to get the tires at the recommended pressure + a maximum of 5 PSI.
One magazine did a test and found major variations in gauges. Get a certified gauge (NAPA carries them for about $35). Then fill a tire using it to verify the pressure, then check your other gauges against it. That's what I do and found every gauge to be different.
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Old 06-03-2011, 06:54 PM   #8
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I'm not a tire engineer, but I would guess that +/- 5% would be fantastic. I would probably accept +/- 10% but would be likely to adjust if the cold gap grew to more than that. Of course, I'm an engineer of another discipline with all the issues that creates in one's personality.

I've gotten mine within 2 psi with a gage, the seen the spread grow to 6-7 psi in use (using a TST TPMS). The sunny side seems to be warmer than the shady side, based on my limited experience with the TPMS (only three trips so far). Seems logical.
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Old 06-03-2011, 07:00 PM   #9
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Actually, I believe they can be different as they should be inflated according to how much weight is on each individual axle. Your axle weights can vary a good bit. If you don't weigh them independently then I would try and keep them within a few pounds difference. Your tire manufacturers web site may give you inflation figures per weight on the tires.
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Old 06-03-2011, 07:25 PM   #10
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First: it is 100% normal to run different pressures in different tires, IN fact it is very rare that you should run them all the same.. Very rare. So the TST insisting they all be the same is .. A problem

As it happens I think even on the same axle different pressures may be indicated

Some folks (NOTE ME) feel inner and outer duals need to be inflated differently.

How do I figure pressures?

Well, Scale the rig, get the load on each wheel (not tire, wheel, duals count as one) go to the tire maker's web site and get the pressure for load chart, Inflate per chart.

This may well result in different pressures on each wheel.

Some folks say if right and left side are different, use the higher pressure.

FORD: used to recommend running the firestone tires on their Explorers 5 psi lower than Firestone suggested,, To give a softer ride... We all know how that worked over don't we.

(Not well, Firestone took the blame but truth be told I think it was a combination of things starting with that 5 PSI, and ending with a pot hole on the freeway, In between was an oil change techincian who failed to re-check tire pressure and a driver who believed new tires do not loose pressure (All tires do) )
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Old 06-03-2011, 08:36 PM   #11
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Holy Shmoly! For Michelin tires go to their web site and read their information. For Goodyear, do the same. For any manufacturer, do the same.

Michelin for example wants the same pressure on all tires across the axle. You use the highest weighted tire as a guide. You use the tire pressure charts as given by each manufacturer.

For 100 PSI a ten percent difference is 10 PSI. If that is 10 PSI drop you could well be outside the bottom limit for the weight. If it is the top limit, 10 psi could be outside the limit of the max pressure indicated on the tire.

There are a lot of "tire" threads out there, with much of the information not accurate. The only accurate source is the manufacturer.

Good luck.
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Old 06-04-2011, 09:31 AM   #12
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I have to admire those who can get...

Thanks for all the interesting replies. I have to admire those who can get their TPMS to read all the same. When it happens with just a few tires I feel very lucky.

When I said the TST TPMS insists on telling me they aren't the same I just meant it read out the values it has which I then process in my brain and my brain tells me I didn't get them all the same. There isn't an actual alarm or anything that tells me when I'm a few lbs off. The alarms are for things like fast pressure loss, high temp etc.
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Old 06-04-2011, 10:10 AM   #13
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Also as you drive the sunny side is normally hotter than the shady side so the pressures go up more on the sunny side.

Oh, as to how close should it be.. I like to put 'em within 5 PSI or target cold.. Of course if I'm doing the inflation bit, I shoot for closer to 0.5 psi from target (Limit of resolution on my pressure guage) but hey.. that's how I'd do it.

If I ever get my cross fires installed I'll be able to do even better.
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Old 06-04-2011, 08:55 PM   #14
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I'm always surprised at the big difference in tire temperature between the sunny side and the shady side so the starting pressure has been changed differently by this factor and certainly will be by others. On the road, during the day, I check tire temperature, not pressure.

JT
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