What he is sating is that you are full of BS! If you had a clue, you would know that.

 

An old trick when driving on crappy or washboard roads, when I used to log I would drop my tire pressure on my truck and instead of crawling down the road a 20mph feeling every bump in the road the tires would "float" over the washboards at 60mph +.

When I got to pavement I would inflate them to the proper 32psi.

My '80 door sticker says 35psi front and rear. You said you had an '82 (or was that BS) go out and take a look at what the door sticker says on that one.

As for your contact experiment goes it proves that you get more of a contact patch with lower psi meaning you are rolling on the shoulders of the tires.

 

There's no "Duh" about it. If the SHOULDER is wearing, that means the tire is ROLLING OVER ONTO THE SHOULDER in turns. That means the tire is being ALLOWED to roll onto the shoulder in turns. And THAT, my friend, is being allowed by insufficient internal tire pressure.

Period.

I'm very sorry, but there isn't much room for debate in the matter. That IS what is happening.

If the tire were properly inflated, it would not look like that. The shoulder would be more square.

WHICH THEY CORRECTED by progressively increasing the recommended tire pressures as the radial tires installed on the C3 Corvette were better understood as the years wore on.

This is clearly demonstrated by the placards being revised in later-year cars.

WHAT THE HECK ARE YOU???!?!?!?!?!?!?!!??!?!?!?!?!?!?!?!?!? ?!?!?!!

Point blank: If you were not a GM Engineer responsible for establishing the tire pressures in the Chevrolet Corvette between the years of 1974 and 1982, you have ABSOLUTELY NO MORE KNOWLEDGE on the subject than any one of us.




Insulting everyone in the forum who is not as awesome as you will not win you a single friend. Anywhere. Anytime. Ever.

I beg of you...please be done with this. You are incorrect in your assumptions, your analysis, and your conclusions.

 

I own three C3's; a red 1968 with a 454" and 4-speed, a blue 1971 with 454" and TH700R4, and my red 1982. Of the three only my '71 is licensed for the road so its my daily driver.

When I was prepping my '82 for it's new Torch Red paint job I accidentally rubbed the numbers off the door sticker but I seem to recall it saying 35 psi maximum pressure @ maximum load. But I have run 20/20 psi starting with it's 3rd set of tires so I could get the maximum tread life. Something you should do too if you weren't so afraid of the monster under the bed.

 

Any time the centers get worn out way ahead of the sides the tires are over inflated and you don't have to be a rocket scientist to figure that out. The OP simply asked why his door sticker said 20 psi and thanks to you guys spouting your 30+ psi nonsense the poor guy is going to learn the hard way when his tires last only half their normal life span.

You guys have continually asserted 20 psi is too low but you have never given any valid reason why its too low. I have been running 20 psi for 20 years without a single bit of trouble because 20 psi is the correct pressure for a C3 when running the big 225 thru 255X60R15's.

 

Can you please explain why you continue to refuse to understand the facts others have presented you. It really appears like you totally ignore anyone's explanation.

 

Obviously ....either you can not comprehend the data and explanations WHY 20 psi is NO LONGER used.....COUNTLESS TIMES in this thread......OR....you have nothing else better to do that TRY to support your views with test that do nothing to support your way of thinking.

Go back and READ post #35...just a ONE example.

You are stuck in this 'myth' that 20 psi is fine. WHEN...the rest of us KNOW that you are now just being obstinate and will not accept the information given. You are so concerned about tread life....but not concerned that your alignment is not correct. Becasue you admitted that it is off. SO...if you are going to try to make yourself be ...'all that'...because I do not know if your know this or not...but you come across as being ( fill int the blank)....well with that written...you better make sure that your cars are up to the task of being...'all that'.....IN every way.

I am ...or WAS.... on your side in being allowed to express your thoughts a views...BUT this is totally ridiculous. HOW can you contradict the tire pressure that is given by GM in the placard in post #35. And if your argument is that your car is not at MAX LOAD....then dropping the tire pressure 15 psi....is NOT logical...when a person with common sense would take into account the side wall deflection issues in a panic maneuver.

DUB

 

YOUR picture of the worn out shoulders on YOUR tire is proof that 20psi is too low.

No-one needs any more proof to know you have no clue what you're talking about.

 

 

If you go down to 1 or 2 psi you will get even more rubber on the road. Try it see if it works.

 

Mods need to put this thread out of its misery.

 

PLEASE!

 

I have yet to read one single valid reason why the formerly safe 20 psi is no longer safe. I have read your ASSERTIONS why its not safe but those are just assertions without a bit of evidence to back them up. You guys have the mindset every tire requires 30+ psi regardless of how heavy the vehicle is or how its operated. The big 255X60R15's have a service factor of 2.0 when mounted on a C3 but I'm sure you don't have a clue what that means.

 

For the 10th time (100th?), bias ply tires are constructed completely differently from radials. Bias ply sidewalls are a lot stiffer thus needing less pressure than a radial.

A service factor, LOL! What in the world are you talking about? Why don't you write what you found scouring in Wikipedia about service factors?

 

Here you go right from wikipedia

The tire contact patch is readily changed by both over-and-under inflation. Over-inflation may increase the wear on the center contact patch, and under-inflation will cause a concave tread, resulting in less center contact, though the overall contact patch will still be larger.[42] Most modern tires will wear evenly at high tire pressures, but will degrade prematurely if underinflated. An increased tire pressure may decrease rolling resistance, and may also result in shorter stopping distances[43] If tire pressure is too low, the tire contact patch is greatly increased. This increases rolling resistance, tire flexing, and friction between the road and tire. Under-inflation can lead to tire overheating, premature tread wear, and tread separation in severe cases.[44]

 

Taken from 4 wheelparts.com web site.
Tire Pressure Checker: Righting the Pressure in Your New Tires
The excitement of getting new tires can leave you giddy in the short term. But, if you expect those new tires to keep making you happy over the long haul, you’ll need to address some practical concerns before it’s too late.

Once you upgrade your tires, the first objective is identifying the correct tire pressure for street driving. To determine the ideal tire pressure for a stock truck, your best resources are the tire inflation placard and the owner’s manual. Unfortunately, these resources don’t help you when you don’t have stock or close-to-stock tires on your truck. Once you put bigger tires on your rig, it’s a whole different ballgame.

Let’s start with two simple ideas. One, the load of your truck is supported by the air volume in the tires. The constant is how many cubic feet of air can hold up a specific weight. What isn’t constant is the size of the container holding the air. Say you have two containers of different sizes holding the same volume of air. In this scenario, the smaller container will have higher pressure than the larger container. That means the larger tire needs less pressure than a smaller tire to carry the same vehicle weight. What many truck owners don’t know is how much less pressure is required. You can calculate this difference using one of four methods.

Let the sidewall tell you
This is the easy, guesswork method, and provides an immediate answer to your question. Find the maximum pressure as shown on your sidewall and let that be your street pressure. It will suffice until you have time to make a more accurate calculation. Note that this usually means your tire pressure will be too high, particularly if the truck is not carrying a load. This is because the maximum air pressure is the right number when the truck is fully loaded. The sidewall should also tell you the load capacity at the maximum tire pressure. That capacity can likely support half your truck’s weight, or more.

The advantage of using this maximum pressure is that it’s safe. You may even get a fuel economy benefit out of it, because your rolling resistance will be minimized.

The disadvantages are related to ride quality, handling, and uneven tread wear. At maximum pressure, you’ll feel every bump in the road. The contact patch will be concentrated in the center of the tire, which impacts the responsiveness of your steering and brakes. You may see faster tread wear in the middle of the tire.

Let the vehicle weight tell you
The tire maker defines the ideal tire pressure for loads of various sizes. You can get the chart for your tire from your tire maker, a tire shop, or online. A less accurate alternative would be a generic chart from the Tire and Rim Association. These recommendations are available in the most popular sizes, but the stated pressures are not specific to your tire.

Weighing your truck. The next step is to obtain the front and rear weight of your truck. Do not estimate or assume that your loaded truck weighs the same as it did when it rolled off the lot. You’ll get a more accurate result by packing your equipment onto the truck, filling the gas tanks, and even inviting some passengers to come with you to the scales.

Scales are typically available at scrap yards, recycling centers, landfills, and, of course, truck stops. You need to determine the amount of weight on your front tires, and then, separately, the amount of weight on your rear tires. If there’s a race shop near you, you may be able to use its portable scales to get these weights quickly.

At a truck stop, you’ll have to take three different weights. First, drive up and stop when only the front tires are on the scale. Record the weight. Next, drive your truck forward until all tires are on the scales. Take another reading. Lastly, drive off the scales so that only your rear tires are being weighed. If you’ve done it right, the sum of the front and rear tire readings should be roughly equal to the total truck weight. You may learn that your truck carries more weight on the front than on the rear. This isn’t that uncommon with pickups.

Next, divide the weight on your front tires by two to get the weight on each tire. Do the same with your rear tire weight.

Finally, consult your load/inflation chart again. Round the calculated weight up to the nearest tire pressure and add another 10%.

Check your results. To check your results, you can measure the tires and add pressure to balance them. Before you measure, make sure your truck is parked on flat ground. Then, along the centerline, measure the distance between the ground and the edge of the wheel on all four tires. As noted, if there are differences, add pressure to even things out. The profiles should be almost the same from front to rear.

The advantage of this method is that you should end up with the optimal pressure. Optimal means you are balancing fuel efficiency, the quality of your ride, handling and braking performance, and uniformity of tread wear.

This disadvantage of this method is that it’s tedious and could give you the wrong answer if you weigh or calculate incorrectly.

Let the chart tell you
If your truck has not been substantially modified, outside of the larger tires, you can use your tire inflation placard and a load inflation chart to identify a tire pressure.

The first step is to find the chart that goes with your stock tires. Locate the pressure that relates to the recommended pressure on the tire inflation placard. Then, look for the weight the tire should bear at that level of pressure. Next, get the chart for your new tires. Look for the same weight there. If that specific weight isn’t listed, select the next highest weight and record the recommended pressure.

Let’s go through an example, using a 2005 F-150HD that originally had 245/70R x 17D Load Range D tires.

According to the tire placard, at maximum load, the tire pressure should be 50 psi in the front and 60 psi in the back.
When you locate that tire at those pressures on the load inflation chart, the rated loads are 2,205 lbs. and 2,469 lbs.
The proposed new tires are 285/70R x 17D Mickey Thompson MTX Load Range D tires. The closest numbers on the load inflation chart for those tires is 2,340 lbs. and 2,540 lbs. These equate to 40 psi and 45 psi, respectively.
If you need to adjust for differences in the pressure or weight between the charts, you can do it using this formula:

Tire weight/tire pressure = Load capacity pounds per psi
You would start with the load and pressure closest to the original tire, adding or subtracting to find the proper adjusted pressure. In this example, the calculation would be:

Original tire: 2,205/50 = 44.1 pounds per psi
New tire: 2,130/35 = 60.8 pounds per psi
To determine how much tire pressure to add, subtract the weight of the new tire at pressure from the weight of the old tire at pressure. In our example, this results in 75, or 2,205 minus 2,130. Next, take that result and divide it by the pounds per psi of the new tire. This equates to 1.23, or 75 divided 60.8. Adding the 1.23 psi to our front tires would amount to 36.23 psi, which we rounded up to 37.

For the rear tires, the old tire had a slightly lower capacity than the new one. In this case, you could go with the 45 psi or let out some air to reach 43.6 psi.

Let the chalk tell you
You can also “calculate” your tire pressure with the chalk method. This involves coloring a section of your tire with chalk to see how much tread is making contact with the ground. Start by finding a flat road surface. Concrete is actually the best choice, but you can also do this on asphalt. Make a mark with soft chalk that goes all the way across your tread. Then, gradually drive your truck forward about 50 feet and then backwards 50 feet.

Analyze the chalk on the tire. If the chalk is only worn off on the center of the tire, reduce the tire pressure slightly and go through the process again. With the adjustment, you should see the chalk wear off more broadly. Keep making tiny adjustments in the tire pressure until the chalk wears off evenly and all the way across the tread.

You will have to complete this process for each of your four tires. Once you’ve found the right street pressure, add 10% to all four tires. Then, measure the tires and add pressure to balance them. As explained above, you need to measure from the wheel to the ground. Start by balancing the profiles of the front tires with each other. Then, balance the front tires again with the rear tires. Always adjust the tires with the smaller profiles by adding air.

The advantages and disadvantages of this method are the same as those involved in the second method above. If you go through the process correctly, you end up with the ideal tire pressure. But, this method is tedious and there’s a reasonable chance that you will make a mistake.

Testing your tire pressure

You should always test out the results of your tire pressure calculations with a heat test. If tires are under-inflated, unnecessary friction and rolling resistance will result. Those conditions generate heat. Manufacturers use tread pyrometers to test tire heat, but this can also be done by measuring the change in tire pressure. Note that this test is mainly for preventing under-inflation.

Do this test on a day that’s warm, but not hot. Ideally, the outside temperature should be mid-70s to low-80s. To set up the test, locate a relatively straight section of highway that’s 13 or 14 miles long. You want to drive at 55 mph, or the maximum speed limit, for about 10 or 15 minutes. The tires should be at resting temperature, so you may need to stop and relax in the shade somewhere to let the tires cool before you begin.

The first step is to measure the tire pressure of your cool tires. Then, hop on the highway and drive at the maximum speed limit for at least 10 minutes. Pull over safely and take another reading of your tire pressure. If the tire pressure has risen less than 10 percent, then you are done. Your tire pressure passed the heat test.

If the tire pressure has risen more than 10 percent, there is more work to be done. This is because a rise in tire pressure indicates a corresponding rise in temperature. For every 10 degrees Fahrenheit, the pressure will rise about 1 psi. In other words, a 10-psi increase equates to a 100-degree temperature change.

A compressor will come in handy here, so you can add air pressure immediately and run your test again. Start by adding a few pounds of pressure to each tire that demonstrated a large pressure change. Then, let the tires cool and take another 10-minute drive.

Note that tires with aggressive tread patterns tend to run somewhat hotter at the ideal pressure relative to street tires. This conclusion is based on observation and experience only, and there’s no conversion formula to quantify this difference. Just make a mental note that if you are driving on mud-terrain tires, they may generate more heat than a tire with a milder tread pattern would.

Once you complete this test, you should also drive the vehicle and pay attention to the handling, particularly around corners. Let your subjective judgment tell you whether the tire pressure is appropriate or not.

Adjusting for loads

You can use the methods described above for any load state. Just remember that if you add or remove weight from your vehicle after testing, the tire pressure will need to be retested as well. You could complete these tests again every time you change the load, but this will obviously be time-consuming. Many truck owners prefer a less exact approach. They will inflate the tires to max load when the rig is fully loaded, and then let out air for unloaded driving.

Pressure changes over time

Most tires drop about 1 psi of pressure monthly. Changing seasons will also reduce tire pressure, usually at the rate of 1 psi per 10 degrees of temperature change. For example, a tire that reads 31 psi at 30 degrees might read 35 psi at 70 degrees. The only way to address these changes is to check your tire pressure often and at regular intervals. You will be rewarded for your efforts with better fuel economy and more even tread wear.

One last note

The last three methods described above require patience and attention to detail. If you can’t dedicate your focus to the process, then use the max pressure noted on the sidewall.

- See more at: http://www.4wheelparts.com/tire-whee....G0mybE3u.dpuf

 

The edge wear is proof that it's too low a pressure. You really need to quit.

 

You know what? I read this entire freaking post and agree with it.

Without my awesome "race car" type temperature gauges in each of the 3 tire zones (inside, middle, outside) to measure tire temperature in various situations, I'd agree with the "temperature rise" method of identifying whether a tire is properly inflated.

There are several items of note:
---The starting point is the placard on the car. In this situation, you simply must start with the 1982 35/35 recommendation as a starting point.
---There is a constant "add 10%" recommendation in numerous locations. This is prudent, especially if the test is being conducted on a not-hot day.
---There is a "disclaimer" to always default to how the car feels. Having a Corvette "feel like a Cadillac" by using inadequate tire pressure to add 'spring' via sidewall flex is not the way it should feel.

This thread has reached an end. There are many different folks trying to talk reason into a single individual using many different facts and angles.

He just doesn't get it. What's worse, he's made it clear he has every intention to be belligerent, argumentative, and insulting of others with no backing other than one single measure...a single measure that has been debunked by his own picture of his own tire.

 

Everyone here understands that statement.....except one.

 

I would be curious as to where these mountains are in Midland TX.
I see you removed your location from your sig.
Was that as accurate as most of your other information?