Tire pressure 1967 Cadillac

[richardbergquist]

I’m sure there are many opinions on this. Follow the manual or have higher pressure? I have 30 psi now.

[35-709]

Bias-plys and radials require different pressures --- if bias-plys, follow the manual --- if radials, which weren't commonly available back then, 30psi would be a minimum, 32 to 35psi is more the norm. 

[rajeevx7]

Bias-plys and radials require different pressures --- if bias-plys, follow the manual --- if radials, which weren't commonly available back then, 30psi would be a minimum, 32 to 35psi is more the norm. 

Yes. Being at the bare minimum, and you get the cushy ride, but have to check it often (remember flipped Explorers?) Size would help too!

I run 38 in all my cars n trucks, 32 in all motorcycles and trailers, and 25 in all the other wheeled motor toys.

[savemy67]

Hello Richard,

How does your car's ride quality feel with the tires at 30 psi?  Have you driven the car far enough to notice how the tires are wearing?

I have posted elsewhere that I think the optimum tire pressure is the one that provides the best ride quality, handling, and tread-wear life.  To a certain extent, these are contradictory criteria.

If you plan on driving your car no more than 500 miles a year, then tread-wear life may not be as important to you as ride quality and handling.  Similarly, if you think you will put only 10,000 miles on your car in the next ten years, there is no point in paying for a tire with a 75,000 mile tread-life warranty.

The '67 shop manual calls for 24 psi front and rear for the original bias ply tires.  As 35-709 says in his post, bias-ply and radials use different pressures.  This is mostly due to how the tires are constructed, and the effect of pressure on handling and wear.  I don't think there is a hard and fast rule about what the minimum pressure should be in a radial, although there is a point at which any tire is under-inflated.

The physics of gas pressure would indicate that some fundamental aspects of a bias-ply tire and a radial tire, both inflated to 24 psi, are similar.  That said, how do inflation pressures of 30, 32, 35, or 38 in radial tires affect ride quality, handling, and tread-wear life?  This is your dilemma which can only be resolved through experimentation.

Here are two modern tires that will fit your '67.  The Cooper Trendsetter SE and the Hankook Kinergy PT.  Both are touring style tires in size 225 70 - 15.  The maximum inflation pressure for the Trendsetter is 35 psi.  The maximum inflation pressure for the Hankook is 51 psi.  Why the difference?  Both are radial tires of the same size.  However, the Hankook comes with a 90,000 mile warranty, so its UTQG (and its materials/construction) is different than the Cooper tire.  All this is to say that one size does not fit all.

I recall that you hit a pothole/dip in the road some time ago.  Imagine hitting that obstruction with a tire inflated to 38 or 50 psi.  The result would probably have been more damage to some of your suspension components.  Adjusting air pressure is an easy chore.  Try adjusting yours with the tires you currently have.  Use the same stretch of road, at the same speed, under the same weather conditions, to see which pressure provides the best combination of ride quality and handling (tread-wear life can only be determined over a longer period).  For your '67, based on the condition of its suspension, alignment, the load you typically have in the car, which tire brand/model/size is on the car, this might be 28 psi, or 34 psi.

One more tip if you decide to experiment with your tire pressures.  Be sure the tires are out of the sun when you check/adjust the pressure.  The tires on the sunny side of the car could be one to two psi higher depending on how long they have been in the sun.  It may take one to three hours for the pressures to equalize depending on the air temperature.

Respectfully submitted,

Christopher Winter

[Scot Minesinger]

The weights/dimensions/suspension of the 1965-70 Cadillacs (and that may well be to 1979) are similar, and for my 1970 Cadillac with radial tires 32-34 psi is about right.  Also, read the inflation info on the tire sidewall, that may offer a limitation that you do not want to get too close to.  As Chris pointed out tread wear is not an issue, mine will be replaced for age before wear unfortunately.