PSI for Radial Tires

[Dan Hiebert]

I can't seem to find any information on what air pressure to put in radial tires on our cars.  A bit of reference, my Beetle has info for both, and shows 2 - 3 psi more for radial tires than for bias-plys.  Would that hold true (or close to it) on our cars?  The shop that mounted my tires just put what the tire's max psi is indicated at, 35.  That'll make for a stiff ride.  I know there would be a difference in ride quality with Dodge's specified psi, but I'm more interested in proper tire wear.   

[plymouthcranbrook]

I run the radials in my 52 4 door Cranbrook  between 33-35 PSI Good ride,  steering easy but secure. 

[laynrubber]

Tires should have a recommended pressure on the sidewall, I go by that. For proper tire wear run the max pressure they list. 

Edited June 17, 2020 by laynrubber

[Andydodge]

When I had the 195/75x15 & 235/75x15 Coker Classic Whitewalls I ran them at 35psi, this was on the 41 Plymouth Coupe, stock suspension but lowered 2" all round....steered and rode well, I was more than happy with the tyres and wear............andyd  

[Sharps40]

Chalk the tire.   Roll the car.   Check the tire for missing chalk.   Adjust PSI up or down.

[DJK]

4 hours ago, laynrubber said:

Tires should have a recommended pressure on the sidewall, I go by that. For proper tire wear run the max pressure they list. 

The pressure shown on the sidewall is max pressure for mounting a tire only. On modern day vehicles, tire pressures are listed on drivers door frame.

[Dan Hiebert]

2 hours ago, Sharps40 said:

Chalk the tire.   Roll the car.   Check the tire for missing chalk.   Adjust PSI up or down.

Roger that - to check that the full width of the tread is making full contact?

[Sam Buchanan]

The load carried by the tires determines how much pressure should be used. A light vehicle won't need as much pressure as a heavy one to provide the tire with proper contact area. The pressure stamped on the sidewall is what is needed for maximum load capacity of the tire which is also stamped on the sidewall.

 

I had 205-50/15 radials on my kit car which had an empty weight of 1500 lbs. I ran 17 psi in the tires for proper contact patch even though "35 psi" was stamped on the sidewalls. My P15 runs with 24 psi in the bias ply tires. The car only weighs 3200 lbs, pumping up the tires to the same pressure (35 psi) as what would be needed for a 4500 lb car is not only unnecessary but unpleasant and may wear the crown of the tread.

 

Hence the recommendation to chalk the tires.....use only as much pressure as needed for proper contact.....and save your achin' back.   ?

[Sharps40]

35 minutes ago, Dan Hiebert said:

Roger that - to check that the full width of the tread is making full contact?

Yes

[kencombs]

2 hours ago, DJK said:

The pressure shown on the sidewall is max pressure for mounting a tire only. On modern day vehicles, tire pressures are listed on drivers door frame.

Yes to that.  The sidewall pressure is also the one that provides the max load rating.  Remember the Ford fiasco with Firestone tires?  The issue was Ford speced a pressure that resulted in too much heat being generated in the interest of ride quality.  W  a  y  below the tires' rating.

 

Chalking the tire as mentioned is a good technique.  A laser thermometer is also a good approach to tuning pressures.  Drive long enough to get tires heated up, stop and check the temp across the tread.  It should be nearly the same, on both outside edges and the middle.  Higher in the middle, too much pressure, higher on the edges, too low. 

Since we often have wheels whose width is not a perfect match  to the modern radials, following normal specs may not be be the best method.  Measure in some way rather than just picking a number.

[greg g]

Been running my 205 75 15 fronts at 32, 225 75 15 rears at 30 with trunk empty 32 when in road trip mode.  Tires wearing evenly got about 50k on the first pair of fronts, Still above the wear bar but mounted in 2004 they were getting more stale than worn out.

[Los_Control]

33 minutes ago, Sam Buchanan said:

The load carried by the tires determines how much pressure should be used

I agree 100%. Technically a tire with zero air pressure is not under inflated, if it is carrying only its own weight as you roll it across the floor.

As you increase the weight you increase the air pressure ... it is the air that caries the load, the tire just contains the air. As we go from a 2 ply bicycle tire to a 16 ply semi truck tire, the casings are simply built better to contain more air pressure to carry more weight.

 

With all that said, I had a small light Triumph spit fire, Think I ran about 24 psi in the rear and 26 psi in front, it had a larger heavy engine.

Any American car I will generally start about 28 psi, look at the sidewalls to see they look correct, check it for ride and cornering. If it feels a little squishy maybe try 30 psi.

 

Another big issue is rim width and tire width, A radial tire typically wants a wider 5.5-6" rim ... if you have a original 4.5,- 5" wide rim, The sidewalls are sucked in and you will never get "proper" foot print. The crown will just be rounded and not flat. and will affect tire wear.

 

Years ago, we had some tires that max was 32 psi, then others were 35 psi ... are all 35 psi now? Simply the casings are built a little stronger and we now have 70k mile tires.

I love the ride of Michelin tires, they give the best ride and best mileage, including semi trucks. They build the shoulders different, they have a wedge built in, gives a flatter foot print while a more rounded shoulder for ride and performance.

So with all the variables, kinda obvious each situation is different, same exact car, try different wheels with different brand tires and will need different air pressure. You will need to experiment.

 

 

[laynrubber]

Huh, I think I learned something today. Chalking the tires sounds like the best method. Disregard my idea and I feel sheepish now. Ummm anyone got chalk.....

[Sniper]

1 hour ago, kencombs said:

Yes to that.  The sidewall pressure is also the one that provides the max load rating.  Remember the Ford fiasco with Firestone tires?  The issue was Ford speced a pressure that resulted in too much heat being generated in the interest of ride quality.  W  a  y  below the tires' rating.

 

 

 

Not true, here's an analysis by a tire engineer.

 

http://www.barrystiretech.com/fordfirestone.html

 

Here's the relevant analysis

 

"But what about the inflation pressure specified by Ford?

To the right is the tire placard from a 1998 Ford Explorer

Notice that the GAWR's (Gross Axle Weight Rating) - the maximum weight the axles were designed to carry - is listed as 2510# front / 2900# rear. The GAWR includes all components that are included in the rear axle as an assembly - the axle itself, the springs and mounting brackets, the hubs, the rims, and the tires.

The load carrying capacity of a P235/75R15 at 26 psi is 1753# - and when we apply the 10% derating because the tire is applied to an SUV (This also applies to pickups, vans and trailers.), the load limit is 1594#, or 3188# for an axle.

That means the designed GAWR is 79% of the rated tire load for the front axle and 91% for the rear axle.

This means the tire was within the limitations as specified by the Tire and Rim Association - the US tire standardizing organization."

[lepic56]

I got this from My brand of tires,,  American Classic.

 

"""With its steel belted construction and polyester cords, American

Classic radial tires have an estimated lifespan of 45,000 miles, as long as the suspension is aligned correctly. All American Classic radials are tubeless tires and all sizes are both DOT and ECE approved. ECE is the certification for the European market. Recommended tire pressure is 35 PSI, and anything below that pressure can lead to inconsistent wear or tire failure.'''

 

I found 35psi a bit high,, but they made the tires,,,  but I really like the chalk system & the heat reading gun,,   I will try both..

 

 

[Los_Control]

16 minutes ago, lepic56 said:

I found 35psi a bit high,, but they made the tires,,,  but I really like the chalk system & the heat reading gun,,   I will try both..

Sounds like you are on the right path.

Above I mentioned,

1 hour ago, Los_Control said:

I love the ride of Michelin tires, they give the best ride and best mileage, including semi trucks. They build the shoulders different, they have a wedge built in, gives a flatter foot print while a more rounded shoulder for ride and performance.

You are running the American Classics, to be honest I never heard of them, curious to hear how they work out .... I assume they are radials that "look" like a original bias ply tire.

Again it is the shoulder area that is built different. Will be more of a straight up and down sidewall to "get the bias tire look" While a modern radial will have the curved shoulders.

 

Right out of the box, you have a compromise to get the best of both worlds, looks vrs ride/performance.

You wont achieve 100% looks and ride. Keep that in the back of your mind while experimenting.

 

I would also assume but have no proof, it may take a bit more air pressure then normal ... 35 psi, for sidewall of this particular tire brand to hold shape ... I assume they did some testing themselves.

 

It is the flexing of the sidewall that causes heat, heat causes tire failure. Pick a 3 or 5 mile route, cold start in the morning,  start with 35 psi, get home and check temp with heat gun.

That should be your base temp. 

Next morning cold, start all over again with a different pressure, maybe 30 psi. Exact same route and check temp again ... Rinse & repeat.

I would not want to vary to much from your base temp, you may find that 35-32 psi is where these tires need to be. ... They simply are not built like the tires on my wife's mini van or my daily driver pickup.  Man they sure look nice!

 

[laynrubber]

The heat gun method might be too complicated for most of us.....I can see it in Formula racing but for the average joe it seems like too many variables. At what point does the tire get heat soaked and give erroneous readings, outside air temp would affect how far to drive before heat soaking. Not driving far enough and not enough tire temp. Seems pretty high tech, interesting enough I am going to try it myself.

[Los_Control]

1 hour ago, laynrubber said:

I can see it in Formula racing but for the average joe it seems like too many variables. At what point does the tire get heat soaked and give erroneous readings

I do not own a heat gun, I have never found a use for one. I imagine they are nice. .... I would not buy one for tires, though I think it may be a good example.

My first post, I claimed I would try different things and then settle where I liked. Then when you replied you may try a heat gun, I assumed you already had one.

 

Point is, you are running a custom tire that will require custom attention,. .... These old cars are not complicated, nobody had fancy tools to keep them on the road. They just did what they did to keep them running. They are very basic. You need to determine yourself best air pressure.

[lepic56]

5 hours ago, Los_Control said:

Sounds like you are on the right path.

Above I mentioned,

You are running the American Classics, to be honest I never heard of them, curious to hear how they work out .... I assume they are radials that "look" like a original bias ply tire.

Again it is the shoulder area that is built different. Will be more of a straight up and down sidewall to "get the bias tire look" While a modern radial will have the curved shoulders.

 

Right out of the box, you have a compromise to get the best of both worlds, looks vrs ride/performance.

You wont achieve 100% looks and ride. Keep that in the back of your mind while experimenting.

 

I would also assume but have no proof, it may take a bit more air pressure then normal ... 35 psi, for sidewall of this particular tire brand to hold shape ... I assume they did some testing themselves.

 

It is the flexing of the sidewall that causes heat, heat causes tire failure. Pick a 3 or 5 mile route, cold start in the morning,  start with 35 psi, get home and check temp with heat gun.

That should be your base temp. 

Next morning cold, start all over again with a different pressure, maybe 30 psi. Exact same route and check temp again ... Rinse & repeat.

I would not want to vary to much from your base temp, you may find that 35-32 psi is where these tires need to be. ... They simply are not built like the tires on my wife's mini van or my daily driver pickup.  Man they sure look nice!

 

 

this what I have on it now ,, it is call American Classic  and made in the USA by Coker Tire Company.  a Coker BFGoodrich.

 

with a Max Load Capacity	1874@35 psi

 

[kencombs]

7 hours ago, Sniper said:

 

 

Not true, here's an analysis by a tire engineer.

 

http://www.barrystiretech.com/fordfirestone.html

 

Here's the relevant analysis

 

"But what about the inflation pressure specified by Ford?

To the right is the tire placard from a 1998 Ford Explorer

Notice that the GAWR's (Gross Axle Weight Rating) - the maximum weight the axles were designed to carry - is listed as 2510# front / 2900# rear. The GAWR includes all components that are included in the rear axle as an assembly - the axle itself, the springs and mounting brackets, the hubs, the rims, and the tires.

The load carrying capacity of a P235/75R15 at 26 psi is 1753# - and when we apply the 10% derating because the tire is applied to an SUV (This also applies to pickups, vans and trailers.), the load limit is 1594#, or 3188# for an axle.

That means the designed GAWR is 79% of the rated tire load for the front axle and 91% for the rear axle.

This means the tire was within the limitations as specified by the Tire and Rim Association - the US tire standardizing organization."

I've seen that before.  And I'll stand behind my take on it:  Designing a system that operates at 91% of maximum load at standard operating temps  is poor engineering.  The actual weight of a 98 expedition, the ones with most of the problems (like mine) is over 5000lb, empty, probably 2200 on the rear axle.  Again, empty, any rear seat or cargo weight would be almost all on the rear + 1/2 the 1st and second row passengers. 

 

Hotter than standard air temp,  OK/TX/AZ pavement temps and running 85(normal intercity speed)0 with a full load?  Way too close to the limit for me.   All the failures I've read about, seen first hand, or seem in pictures all showed the classic run low/overheat characteristics.

[kencombs]

5 hours ago, laynrubber said:

The heat gun method might be too complicated for most of us.....I can see it in Formula racing but for the average joe it seems like too many variables. At what point does the tire get heat soaked and give erroneous readings, outside air temp would affect how far to drive before heat soaking. Not driving far enough and not enough tire temp. Seems pretty high tech, interesting enough I am going to try it myself.

Not high tech, complicated or expensive.  And if you work on cars/trucks/tractors much you should treat yourself to one.  Good for diagnosing radiator, water pump, heater core and a lot of other issues.   I got mine from harbor freight for around 25 bucks with a coupon.

 

Erroneous readings are not an issue because you don't really care about the actual temp in our discussion, only the temp differences in various spots.

[Dan Hiebert]

If I recall correctly, the issue with Firestones on the Fords was that they were being run underinflated.  Since someone had to be blamed, the higher courts in a class action suit decided the majority percentage of fault was with Ford and Firestone, instead of the owners, although some lower courts did find fault with owners on some individual cases.  Firestone bore responsibility because the tires shouldn't have failed like they did, even being run underinflated.  This is what led to the nitrogen in tires thing, not better mileage.  Tires stay inflated to PSI better with nitrogen because the molecules are bigger.  My Beetle calls for 18 psi on front, and 27 psi on the rear, but the car only weighs 1900 lbs. (2100 with me in it).  When I had new tires mounted on it a few years ago, the shop put the max psi in them, and the car rode like an ox-cart until I put the correct psi in the tires.  Our D24 weighs a tad less than 3000 lbs, which is why I'm interested in whether the 35 psi is appropriate , because the tires won't need to be inflated for max load (they're Coker Classics), which I haven't checked to see what that is...yet.  I'll do the chalk method to start, if that works out I won't fret about it any more other than maybe doing some calculating out of curiosity...even though I don't like math very much.  Very interesting conversations here, thanks for all the interest and responses! 

Edited June 18, 2020 by Dan Hiebert