Tire Pressure

[Rick Biarritz]

Replacing the tires on my 83 Eldo.  The old ones (not stock) say inflate to a max of 44 PSI.  I have NEVER seen that before; I have always seen 35 PSI.  Is this significant?  Is it some real old design or something?  A Cadillac thing?  Thanks.

[Dave Shepherd]

Unless they are c or d load tires, no way, read the tire again it says max load rating at 44 psi, no?  That is not recommended inflation pressure.  If they are std load range 35 is it.

[Otto Skorzeny]

Dave is in general right but you'll have to be the judge. Tire composition, construction and materials change rapidly. The same brand and model tire may even change from year to year and have different specs.

Under-inflated radial tires are more prone to wacky, dangerous handling than bias ply tires ever were. Anyway, many of the new tires are specifically designed for the higher pressures and will not perform properly if under inflated.

If you put them only to 35, check the handling and wear patterns carefully and frequently to make sure they don't need more air. Someone else on this board recently bought radials that came with an explicit warning to inflate to 41psi.

[Rick Biarritz]

It sort of doesn't matter 'cause I'm putting Michelins on them tomorrow, but I was just curious.  I will read the tire again just to be sure.  Thanks.

[The Tassie Devil(le)]

Tyre Pressures are a cause of concern for those people that don't know what they are doing.

Run too high a pressure when the road is wet, and disaster can happen, and run too low a pressure in the dry can be as bad.

I always run my tyre pressures higher than that recommended because I know what I am doing.

I have found that the Tyre Pressures stated in Owners Manuals are there for the basic Driver, and for the optimum comfort in operating the vehicle.

Bruce.

[Otto Skorzeny]

My girlfriend's car has tires that say 44psi and that's what we keep them at.

[Otto Skorzeny]

The owner's manual recommendations only can be applied to the tires the car was originally equipped with. On a 50 year old car the owner's manual is meaningless for any tire you buy for it today -even the repro bias plys. They are all made differently and from different materials than the originals.

[Steve W]

So, if our owner's manuals are out of date, how do we know what tire pressure we should run for our cars? For instance, my car has relatively new Pirelli P4400s (they came with the car)...on a 68 CDV...

[The Tassie Devil(le)]

With a vehicle that originally came equipped with Cross-ply Tyres, installing Radials would immediately dictate an increase of 4 psi as a minimum, and up to 10 psi as optimum.   

Sometimes the higher pressures will give a harder ride, but handling will be improved.

Take the 1972 Cadillac Eldorado for instance.   The Manual recommends 24 psi in the Front and 17psi in the Rear.   Boy, I put 35psi in the Fronts and 30 psi in the Rears with the change to 235 x 75 x 15 Radials.

Bruce.

[Otto Skorzeny]

Steve, you have to go by the pressure recommended by the manufacturer printed on the side of the tire. It will say something like "inflate to 35psi max" or something similar.

With radials you are going to want them to be very close to the maximum recommended pressure for optimum wear and handling and gas mileage.

[STDog]

In addition, the best pressure depends on the tire load range and the weight on the tires.

The same tires on a 3000lb car will need less pressure than if installed on a 4500lb car.

If the rear is much lighter than the front (like the Eldo Bruce mentioned) then you need more pressure in the front.

You might find a pressure to load chart from the manufacturer, but they are not real common and hard to get for low end tires.
Perelli might have one available though, at least for they top line tires.

[35-709]

So, if our owner's manuals are out of date, how do we know what tire pressure we should run for our cars? For instance, my car has relatively new Pirelli P4400s (they came with the car)...on a 68 CDV...

Should be able to ask any reputable tire dealer.  But without getting into specifics radials require more pressure.  The "max" inflation pressure listed on the sidewall of the tire is just that --- the maximum the tire should be inflated to when carrying it's maximum rated load --- you do not necessarily want that much pressure for everyday driving.  On my '71 Sedan deVille I ran 35#'s all around on its 235x75-15 radials.
Geoff   

[bcroe]

I have another plan.  I look to see how tires wear out.  If they wear at the edges, I put in more pressure.  If they wear in the middle, I use less.  When they wear straight across, I have it just right.  My ideal pressure will vary between front & rear, or for different vehicle weights.  In general, it is a lot higher than the original vehicle recommendation.  Bruce Roe CLC # 14603

[Misfit]

It appears Bruce has it down pat. Generally the recommended Max pressure is the one you want to use as a really close guide.

Do you have a big front wheel drive car with all the wieght on the front end? Increase the pressure. In my 76 Eldorado I run 40psi in the front and 32psi in the rear. Nice even wear across the board.

MisFit

[eam32]

It appears Bruce has it down pat. Generally the recommended Max pressure is the one you want to use as a really close guide.

Do you have a big front wheel drive car with all the wieght on the front end? Increase the pressure. In my 76 Eldorado I run 40psi in the front and 32psi in the rear. Nice even wear across the board.

MisFit
[/quote  I hope you're not driving that car with 40psi in the front tire more that 40 MPH for any length of time.  BE CAREFUL!  EAM

[STDog]

 I hope you're not driving that car with 40psi in the front tire more that 40 MPH for any length of time.  BE CAREFUL!  EAM

What? If the tires are rated for the pressure what's you worry?
Especially on for a front heavy car like a 76 Eldo.

[Frank N. Stein]

44 psi is typical of a light truck tire, maybe thats whats on it? do they say LT before the tire size? I saw a few like that on some back when I was a tire guy. I like to run mine @ 32 in the winter, 35 in the summer. but thats just me.

[eam 32]

The factory PSI tire pressure for 76 Cadillac de Villes and Eldorados for bias tires was 28 PSI at the most so add a couple more would bring you up to 30 PSI which has to be adequate for riding comfort and safety. As you drive the pressure can go up another 5 so that would get you to 45, not for me.  TO EACH HIS OWN!  GOOD LUCK!  eam

[STDog]

The factory PSI tire pressure for 76 Cadillac de Villes and Eldorados for bias tires was 28 PSI at the most so add a couple more would bring you up to 30 PSI which has to be adequate for riding comfort and safety.

You missed the whole point earlier. The pressure ratings for bias ply tires are meaningless for radial tires.

30PS is way to low for most radial tires on a big heavy vehicle. If you are driving a 70s Cadillac with radial tires and 30PSI then they are probably under inflated.

Using the wear pattern is the best way to tell.

As you drive the pressure can go up another 5 so that would get you to 45, not for me.

All tire pressure ratings are for cold pressure. They take into account that increase. So, if the max listed on the tire is 42, that's a cold pressure, and it is safe when it heats up.

[Frank N. Stein]

It sort of doesn't matter 'cause I'm putting Michelins on them tomorrow

Mastercraft AS IV's are a really nice looking all season radial, black wall or white wall.