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tire presure

Old 12-02-2015, 10:40 AM
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White 64
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Default tire presure

I'm buying a 74, the door jam sticker tire pressure is 20psi, why so low?
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Old 12-02-2015, 10:55 AM
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Ongoing debate.

In the most general of senses, the lighter the car relative to the tire width, the lower the pressure that is necessary to have the right "shape" to the contact patch.

The Corvette is not a particularly heavy car and it has pretty beefy tires on it.

This is the same principal that allows dragsters to run REALLY low pressures in their slicks. And big 4x4's to run REALLY low pressures in their flotation tires.

Personally, I'd have a little bit of an issue running 20 psi. I think I've dipped as low as 26. But listen to what YOUR tires are telling you.

Feel them after driving them. If they're warm, more pressure.

If both shoulders are wearing, more pressure.

If the center is wearing, less pressure. Or fewer burnouts.

Last edited by keithinspace; 12-02-2015 at 10:56 AM.
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Old 12-02-2015, 11:02 AM
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Originally Posted by White 64 View Post
I'm buying a 74, the door jam sticker tire pressure is 20psi, why so low?

I have been running 24 psi front and 20 psi rear for the last 28 years because I know the 255X60R15's are very large cross-section tires for the minimal load they're carrying.
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Old 12-02-2015, 11:47 AM
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The prevailing information at that time was that those were the right tire pressures front and rear for the tire technology in the early 70's for that vehicle. Modern radial tires handle/ride/wear better at around 30 PSI cold.
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Old 12-02-2015, 12:13 PM
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Originally Posted by capevettes View Post
...for the tire technology in the early 70's...
That's really the deal.

I've recently had some tire issues on 2 of my cars, including my Corvette. In resolving them, I got to see what 20 PSI looked like with my BFG 235's.

Didn't like it. Just too much sidewall bulge. With that much bulge comes heat and also comes less pressure in the center of the contact patch.

30 PSI, plus or minus, is where I like to live.

And I run a lot more pressure in the front of my Subaru which has the big H6 engine hanging 2 feet in front of the front wheels.

Like I said...ongoing debate on this particular issue.

Well...not much of a debate. I've only seen/heard one vocal dissenter.

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Old 12-02-2015, 12:21 PM
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Originally Posted by capevettes View Post
The prevailing information at that time was that those were the right tire pressures front and rear for the tire technology in the early 70's for that vehicle. Modern radial tires handle/ride/wear better at around 30 PSI cold.

Oh, so the "modern" 255X60R15's are made very different from the "old" 255X60R15's? No, the 255X60R15's are rated for 1875 pounds @ 35 psi but when only supporting 900 pounds they only require half the pressure; 17-1/2 psi. But to compensate for the higher loadings going over bumps and around corners the G.M engineers decided on an even 20 psi.

Inflate your big 255X60R15's to 30+ psi and you'll wear their centers out before they have reached half of their expected life.
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Old 12-02-2015, 12:35 PM
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Please sir. You believe what you believe. ALL the rest of us believe what we believe.

If there is a single other person that would take up your flag and make this a debate, I'd be a VERY happy person. Then at least it would seem like a more reasonable position.

A story:

I have something I tell my son who sometimes think EVERY quip that leaves his lips is hilarious: Just because you say it, doesn't make it funny.

Replace "funny" with "true" and we'd be onto something.

To the OP:

Please continue with the thread and ask whatever you like. You may not get the input or amount of feedback you'd like on this particular subject because it was BEAT TO DEATH on a separate thread very recently. Many wonderfully helpful contributors on this forum simply won't engage in another endless/pointless debate on the subject...

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Old 12-02-2015, 01:19 PM
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Originally Posted by toobroketoretire View Post
I have been running 24 psi front and 20 psi rear for the last 28 years because I know the 255X60R15's are very large cross-section tires for the minimal load they're carrying.
What tires are you using on your cars?
Make model Size

Last edited by Kacyc3; 12-02-2015 at 01:19 PM.
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Old 12-02-2015, 02:11 PM
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I would suggest that those who feel this topic is not interesting or not enough of a debate, to not post in the thread, and move on to one that is interesting. If you want to help the OP, rather than telling him it's been discussed at length before, provide him with the link to that thread so he can read it for himself. He posted this thread because he had a question.

I'd also caution against personal attacks. If you do not feel an argument has merit, debate it or ignore it.
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Old 12-02-2015, 02:58 PM
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Originally Posted by Kacyc3 View Post
What tires are you using on your cars?
Make model Size

All three of my C3's are running 255X60R15's. They are rated for a maximum load of 1875 pounds @ 35 psi so when mounted on a 7500 pound vehicle the proper pressure would be 35 psi. But when mounted on a 3750 pound vehicle only half the pressure is required to support that weight and when mounted on a 1875 pound VW bug only 10 psi would be required.

So yes, your door sticker is 100% correct because the mid to late C3's came with their very large cross-section 255X60R15 tires that are rated for double the C3's weight.
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Old 12-02-2015, 03:16 PM
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Default 1974 Tire Size

Originally Posted by toobroketoretire View Post
All three of my C3's are running 255X60R15's. They are rated for a maximum load of 1875 pounds @ 35 psi so when mounted on a 7500 pound vehicle the proper pressure would be 35 psi. But when mounted on a 3750 pound vehicle only half the pressure is required to support that weight and when mounted on a 1875 pound VW bug only 10 psi would be required.

So yes, your door sticker is 100% correct because the mid to late C3's came with their very large cross-section 255X60R15 tires that are rated for double the C3's weight.
The 1974 model did not have 255x60R15 as an option. The only tire from the factory was GR70x15 which equates to 225/70x15 which is not a large cross-section tire. The pressure GM recommended at the time was a compromise number based upon the car's level of technology and the tire's level of technology. Today with a modern radial anything less than 30 is ill advised. I was looking for the previous thread that seemed to be endless but can't seem to find it. Perhaps you can post the link so this thread doesn't follow the same path and quickly ends.
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Old 12-02-2015, 03:24 PM
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Default More On Tire Pressure

AA/Fuel dragsters and funny cars only run 7 psi in their rear tires. Why so low? Because the huge tires aren't supporting very much weight! A C3 running 255X60R15's @ 20 psi will have much better traction, will ride a LOT smoother, and tire life will be almost double.

Tire bulging at low pressures: In a recent test I found the big 255X60R15's were 11-1/32" wide @ 35 psi. I dropped the pressure in 5 psi increments and found no significant bulging took place until the pressure had dropped below 5 psi. The point is you can't tell how much pressure your tires have in them by their appearance alone because they still retain their shapes way below their recommended pressures.
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Old 12-02-2015, 03:53 PM
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Originally Posted by toobroketoretire View Post
All three of my C3's are running 255X60R15's. They are rated for a maximum load of 1875 pounds @ 35 psi so when mounted on a 7500 pound vehicle the proper pressure would be 35 psi. But when mounted on a 3750 pound vehicle only half the pressure is required to support that weight and when mounted on a 1875 pound VW bug only 10 psi would be required.

So yes, your door sticker is 100% correct because the mid to late C3's came with their very large cross-section 255X60R15 tires that are rated for double the C3's weight.
What brand of tire?

Ya know Ford did the same thing with the Explorers and the Firestone tires. Going against Firestone's recommendation on air pressure Ford said to run 26 PSI in the tires causing excessive heat which damaged the tire. So the question becomes have you contacted the manufacturer and asked what the lowest recommended air pressure is for your tires?
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Old 12-02-2015, 04:04 PM
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an actual test with pictures showing the effect of low tire pressure, 25 psi, on contact patch area
http://www.tirerack.com/tires/tirete...e.jsp?techid=3
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Old 12-02-2015, 04:26 PM
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Originally Posted by White 64 View Post
I'm buying a 74, the door jam sticker tire pressure is 20psi, why so low?

If that 20 psi is for a 255X60R15 its the correct pressure for your C3.
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Old 12-02-2015, 04:44 PM
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Default Ford Explorers/Tire Failures

Out of curiosity I just Googled "Ford Explorers/Tire Failures" and found the tires were failing because the ignorant owners were adding 500+ pounds of cargo and letting their tire pressures drop below 20 psi. So it was an owner problem; not a tire problem.
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Old 12-02-2015, 04:53 PM
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Originally Posted by toobroketoretire View Post
Out of curiosity I just Googled "Ford Explorers/Tire Failures" and found the tires were failing because the ignorant owners were adding 500+ pounds of cargo and letting their tire pressures drop below 20 psi. So it was an owner problem; not a tire problem.
one of the many articles I found.
http://content.time.com/time/busines...128198,00.html
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Old 12-02-2015, 07:03 PM
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Believe what you will. 20psi will allow the tires to roll and move around more which makes the car feel lousy to drive and uses more fuel. It's based on the tire tech and what was believed to be best back then. You won't find any new cars of a similar size and weight using such low pressures.

I run between 28psi and 30psi cold in a 305/30R19 tire on a 3300lb car to get even tire wear. I run 32psi cold in a 375/30R19 tire on a 3300lb car to get even tire wear. I've had a number of pickups that are mostly driven unloaded and I have never lowered the air pressure in the rear tires while carrying no cargo, yet I've never had significant center wear on the tires.

So, I just saying the same as almost everyone else, claiming 20psi is required to maintain even tire wear is nonsense. Try to find a single reputable source of tire information which says the pressure which should be used is calculated by taking the rated pressure on the tire and multiplying by the proportion of the weight rating used. You WILL NEVER come up with a source because it's simple made up.

Last edited by lionelhutz; 12-02-2015 at 07:04 PM.
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Old 12-02-2015, 07:22 PM
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Thanks' guys, I'll start with 30psi!!
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Old 12-02-2015, 07:22 PM
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Yeah, well I run blah blah blah in my tires and so should you. This is why you go to a doctor when you're sick; not a forum. If your door jamb sticker tells you to run 20 psi in your 255X60R15's then run 20 psi. The pressure needed in tires is not a matter of majority rules but rather what the vehicle manufacturer calls for.
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