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Old 09-29-2019, 12:31 PM   #1
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Default Psi

Iím planning a long trip with the 40 coupe. My radials say max psi 52. Iím thinkin putting 40 psi. Make sense?
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Old 09-29-2019, 12:41 PM   #2
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Default Re: Psi

depending in the vehicle but anything between 32 and 40 PSI would be ok.
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Old 09-29-2019, 12:42 PM   #3
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Default Re: Psi

Caution is required if you are using stock rims with radials, particularly if you are driving at highway speeds for long intervals, as tire temperature rises considerably from tire flex and road surface temperature. The sidewall info usually states maximum pressure for use on rims designed for radials. Thirty five to 40 sounds reasonable for non radial rims.
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Old 09-29-2019, 12:50 PM   #4
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Default Re: Psi

Not stock rims - new tires - side wall says 52 psi
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Old 09-29-2019, 02:23 PM   #5
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Default Re: Psi

Doesn't that say max pressure and with a load rating? My new car recommends 35 psi but the max pressure listed on the tires is much higher than that.
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Old 09-29-2019, 02:24 PM   #6
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Default Re: Psi

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Doesn't that say max pressure and with a load rating? My new car recommends 35 psi but the max pressure listed on the tires is much higher than that.
Yes it says 52 max. I put in 40 psi.
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Old 09-29-2019, 02:54 PM   #7
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Originally Posted by 30-9 View Post
Iím planning a long trip with the 40 coupe. My radials say max psi 52. Iím thinkin putting 40 psi. Make sense?
What the maximum a tire is designed for has very little to do with the application of that tire.
That depends on a number of things. Position of that tire? Weight of the car? Chassis design? Wheel design?
It sounds as if you have some kind of Frankenstein car there. So I suppose you should put any amount of air in the tires that makes you happy and remaining safe.
Oil in a fan that doesn't require oil... radials....some of the issues I suppose a fellow has to resolve when building such an animal.
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Old 09-29-2019, 02:57 PM   #8
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Doesn't that say max pressure and with a load rating? My new car recommends 35 psi but the max pressure listed on the tires is much higher than that.
Of course you are correct.
Our Aston has tires with a maximum of 70PSI. Specs require 42# in the rear and 36# in the front. Unless we plan on extended speeds of over 100MPH. Then we are to increase all by 4#.
Gosh, I can't imagine how that car would handle with 70# in the tires.
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Old 09-29-2019, 03:01 PM   #9
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Of course you are correct.
Our Aston has tires with a maximum of 70PSI.
Gosh, I can't imagine how that car would handle with 70# in the tires.

Quite likely like you were driving on snot, if not on ice! What color is that Aston? DD
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Old 09-29-2019, 03:06 PM   #10
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Default Re: Psi

I would have thought somewhere in the mid 30s was plenty for a 40 coupe. Am I way out of line?
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Old 09-29-2019, 03:07 PM   #11
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I would have thought somewhere in the mid 30s was plenty for a 40 coupe. Am I way out of line?

I like the way YOU think, Mart. DD
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Old 09-29-2019, 06:58 PM   #12
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Default Re: Psi

maybe a bit less than 30?
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Old 09-30-2019, 02:10 AM   #13
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Default Re: Psi

Tires can be complicated but, in general, 32 psi, will be safe in most cases, unless your car or truck, is pretty heavy. By that, I mean 5000+ lb.

A picture of your side wall marking, will give the info, to use. At 52 lb, it sounds like it would be a 6 ply rating (C) and more of a truck tire. Most car tires are 44 lb or less. My trailer tires are 14 ply (G) rating and are about 120 psi. Ply rating and load rating are different (the numbers in the examples below). Higher ply ratings mean the tire can take more pressure and therefore more load, because of it's higher pressure.

Look at the size marking, a example would be 215/60/16 62P (62 = 584 lb per tire) (p = 93 mph). 215/60/16 100V would be, 100 = 1800 lb per tire and V = 149 mph. The numbers and letters are independent of each other so, you could have any number and letter combination, like 62V. The letters go as high as Z (186 mph)

Those figures will be at the rated tire pressure on the sidewall. There is a chart that will tell you the pressure you need for the load you have, below those figures and a min pressure you should use.

When I raced, with a 1150 lb car (including me), with bias ply tires and 60% of the weight on the rear, I ran max pressures of 12 lb front and 20 lb rear, at speeds to 150 mph and corners like Riversides turn #9 at 130-140 mph. My last race car was 850 lb, with me in it and the tire pressures were max 10 lb, all round. Both cold pressure. Race tires are made to work in the 160 - 210f range. At 220+ they blister and at some point above that, they come apart.

Tire failures come from tire flex and that causes heat, after about 250F, rubber starts to lose it's strength and no longer holds things together and that is when the tread and side walls fail. Ever put your hand on a blown out tire? That said, if the tire is constructed and molded, properly. Most are these days. Firestone was horrible, in the '60's and maybe, until the Japanese bought them. I stopped buying them at that time.

Generally, I run 24 - 26 on a 2500 lb car (240SX) and 28 - 30 on my Mercury Marque. I do that for better traction.

If your ever in doubt about the pressure your running, drive for about 20 min, stop and feel the sidewalls, if there warm to the touch they need more pressure. If they are hot, slow down and increase pressure, ASAP!

I hope I've helped, rather than confuse. I can tell you more, with a picture of the markings.

Last edited by frnkeore; 09-30-2019 at 02:23 AM.
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Old 09-30-2019, 09:06 AM   #14
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Default Re: Psi

Tire pressure for radials is different than for bias ply. Radials generally run less air than the bias ply. Sidewall aspect ratio and normal profile are determined by load on the tire and air pressure to counteract that load. A truck will need more pressure than a light car will due to the possible loading of the truck versus the car. Speed rating will also have an effect such as for low profile aspect ratios for high speed.

This is a good site: https://www.tirerack.com/tires/tiret...jsp?techid=194

On a light body car with a normal aspect ratio radial tire the pressure can vary from 28 psi to 32. If a trailer is going to be pulled then the psi can be higher to suit.
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Old 09-30-2019, 10:25 AM   #15
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Default Re: Psi

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Originally Posted by rotorwrench View Post
Tire pressure for radials is different than for bias ply. Radials generally run less air than the bias ply. Sidewall aspect ratio and normal profile are determined by load on the tire and air pressure to .........
I find this at odds with what I see on my cars. The sticker on the inside of the glove box door on my '51 specifies the tire pressure to be 26 to 28 psi (I will get the precise figures when I go to my shop). This car obviously came with bias plies when new. Every car I have seen running radials as OEM has a sticker specifying something in the low thirties.

Just an observation.
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Old 09-30-2019, 10:42 AM   #16
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Default Re: Psi

I seem to recall 24 psi recommended for the early fifties Fords on bias ply tire. I've run up to 30 psi in the steel belted radials I have on the original Mercury rims but its like running on solid steel wheels unless you are perfectly smooth pavement. I don't do any long extended mile runs so heating of the sidewalls is not an issue. For long runs at high speed I might want to go up over 30 psi and then check the temp of the sidewalls.
Edit: 53 manual advises 26 psi front and 22 rear on sedans.

Last edited by RalphG; 09-30-2019 at 10:48 AM.
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Old 09-30-2019, 11:28 AM   #17
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Default Re: Psi

If memory serves me correctly, that max air pressure is just that "MAX AIR PRESSURE". Which is usually reserved for seating beads when installing tires AND when the vehicle is going to loaded to its maximum capacity.
Part of what you want to look for is "contact patch" of the tire and the road. A vehicle with just one occupant and no add weight wouldnt need "max air pressure" I wouldnt think. However a vehicle with all the occupants it can hold and all their added baggage just may need "max air pressure".
thats just my way of thinking about it
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Old 09-30-2019, 11:39 AM   #18
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"RalphG" - Yep, I believe that's what mine says, as well. I will admit I was surprised at how low they were when I first saw them.
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Old 09-30-2019, 02:08 PM   #19
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Default Re: Psi

This is a example of a 62 Standard Load (SL) tire and the amount of load load it can carry for the amount of inflated pressure. There is also LL (Light) & XL (Extra) rated tires.
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Old 09-30-2019, 02:35 PM   #20
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Default Re: Psi

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Originally Posted by frnkeore View Post
This is a example of a 62 Standard Load (SL) tire and the amount of load load it can carry for the amount of inflated pressure. There is also LL (Light) & XL (Extra) rated tires.

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