I just put 4 Cooper cobras on my 69 vette and the manual says 24 psi, I believe that to be bias ply. Would like to know what other owners inflate their tires to.

 

With my '69BB, I go 28 front and 26 rear with the 255/60/15 Coopers.

 

24psi is definitely for bias ply tires. Radial tires generally have an absolute minimum safe pressure of 26psi.

What size are your Cooper Cobra tires?

 

225 70 15

 

225 70 15

 

Hello, I don't think you can go wrong with 28-30 psi. If you don't like the ride at 30, lower it to 28. Regards, Low/Sporty

 

I run 26 to 28 on my cooper cobras. 225/70-15 front and 155/60-15 rear. Some folks run as much as 32. Kinda depends on the ride you want and the traction you need.

 

I think it's actually P225/70R15. The "P" is significant.

Ok, for that size on a C3 Corvette, rim width matters. If you have 15x8 wheels, 30 is the minimum cold pressure. On any other car, those tires on wheels that wide are not recommended at all. And I don't have a max load number for those tires on 8 inch wide rims at any pressure, just GMs recommendation of 30 psi for the (late years) C3 Corvette.

On a 15x7 wheel, you can safely run that size at 26psi for a load up to 1,521 pounds per tire (which is more than any single wheel carries on any C3). Add 1 or 2 psi for 7.5 inch rim width.

 

Radial tires need higher pressure then bias, but 26 psi is lowest given in US, but European official calculation, wich I got hold of end 2007 and went running with, says not to go lower then 1.5 bar is 22 psi.
And to my opinion its because the old american formula was that bad, that lower then 26 psi calculated gave to much more deflection that overheating of tire at normal speed could happen.
The European calculation, wich US stepped over to 2006, but only for P-tires, gaves lesser deflection in the lower pressure, so can go as low as 22 psi, before deflection gets to much.

​​​​But it was written, that lower would give tire getting from the rimm. But nowadays most rimms have 2 humps ( 2x PANG when mounted) to prevent that.

If you can give me axleweights acurate, and max speed you use, and wont go over for even a minute, together with next of tires, I can calculate a safe highest pressure, with still acceptable comfort and grip for you. Determining the axleloads is the most tricky part in it all, estimating is dangerous.

From tires next, and is given on sidewall:
1. Maximum load or loadindex.
2. Kind of tire ( standard load or XL/reinforced / Extraload) to determine the reference-pressure.
3. Speedcode, less important, have my own simpler and better system for that.

 

35 psi on my Cooper Cobras on an 82- rides a little firm but very responsive and tracks well on rough pavement.

 

If your going on the ‘69 owners manual they had bias ply tires then which have little in common with modern radials.
Lower pressure gives a softer ride but will wear tires faster and may even damage them causing the belts to shift if its too low. I have Cooper Cobras on mine and max pressure is 45psi. I run 40-42psi in mine. At that inflation you’ll get better life out of the tires, better fuel mileage​​​​​​, better handling and the only down side is a firmer ride which you may or may not like. If your car has manual steering like mine fully inflated tires are easier to turn at low speeds too.

 

I'm with you so far.
Too low being defined as what? by whom?

If the tires are underinflated (for the load they are carrying), then yes, they will wear faster, and wear more at the shoulder area than in the center of the tread, and can be damaged.

If the tires are overinflated (for the load they are carrying), then they will also wear faster, and wear much more at the center of the tread than along the shoulders.

Optimum tire wear is when the tires are properly inflated for the load they are carrying, and that's determined by the vehicle weight (and generally indicated on the vehicle tire pressure and GVWR placard, usually with some added pressure for a "safety factor" and on the assumption that tires will be neglected and fall 10% to 20% below that pressure before anyone notices).

I'm not sure which size you have.

For the 225/70R15 size, that's the correct inflation pressure for a 6,500-7,000 pound vehicle, or maybe for a truck axle with 3,300-3,500 pounds GAWR and a full load in the truck. For a C3 Corvette, that's grossly overinflated.

And for the 255/60R15 size, add 500 pounds to the vehicle weight or 250 pounds to the axle weight in my previous statement for that tire pressure.

Only the bolded items are true. Overinflating a tire will give better fuel mileage. It will also make it easier to turn the steering wheel at low speeds. But if that's your primary concern, you're better off going with some drag "skinnies" on the front wheels.

Overinflated tires will lengthen stopping distance, reduce grip for cornering, requiring lower speeds in corners, and overinflated tires wear out the center of the tread much more quickly than properly inflated tires.

I'd stand with the recommendation of 30PSI for the OP's tires. That was the recommended pressure for that tire size in later C3 models when it became a standard size for the car. I might guess that there's probably at least 10% of "safety margin" in that, and for a softer ride and more grip (but a little more "squishyness" as well), you might be able to drop a couple PSI. Unfortunately, GM went "off the standard charts" with the wheels, so I don't have actual engineering data to say that it's safe to go below 30PSI with that size on an 8" wide wheel. On a narrower wheel (7.5" or 7"), that size on a C3 would be safe down to 27PSI, still with an adequate margin of safety.

With the 255/60R15 size on any C3, it's definitely safe to go as low as 26PSI, and unless I was going on a long highway trip (when I might want more MPG and be less concerned with a harsher ride), I'd run that size at 26PSI most of the time.

Story time:
Just a couple weeks ago, someone brought me an older pickup truck complaining that "the rear end is shaking." That truck (an older Dodge Dakota) rode on 255/70R15 tires, which are huge tires for such a small truck. He had been running the tires a couple PSI below the sidewall maximum, 44PSI, but his tires had 2,205 pounds load capacity (4,400 pounds for the rear axle) at any pressure over 40PSI, and he usually had an empty bed in that pickup truck (I'd guess under 2,000 pounds total riding on the rear tires). At first glance from walking around the truck, you'd see good tread on the tires. There was 6/32nds of tread at the outer groove and shoulder blocks. But there was no measurable tread at the center of his rear tires, and he had worn through the belts and cord on the right rear in an area about an inch wide, all the way around the center of the tire. The thing was bulging from one of the places where he had completely worn through the second steel belt. That was the new shake he was feeling. That's the kind of damage you get when running the tires overinflated.

All the tires on that truck were completely worn out in the center. I don't do tires, so I sent him to the best tire place in town, and I also showed him the tire placard in his truck, which shows 30PSI all around for "light load" and 35PSI front, 40PSI rear for "full load". The tire pressure charts I have say even 30PSI was high for the rear tires on his truck with an empty bed. That pressure in those tires would support 3600 pounds on the rear axle. He'd get better tire life at 28psi in the rear tires, and a better ride, and better handling and better braking performance.

 

30 psi.....
you can do an experiment to see what your driving style and car likes. Under inflating tiires will give your car a squishy washed out feel and its similar to driving a gigantic old car with worn out shocks. Think late 60s, monterays, monteclaires, LTDs, cadillacs, etc.. it will sway and wash in th eturns giving you under steer in high input initial part of the turns

start at 25 psi in all four, do a road course in your neighborhood that you are very familiar with and has low pedestrian traffic in case you lose it, stay around 40 mph as you dont need to go overly fast to experience the wash of a soft tire. design your test area to have some slow speed and high speed turns and some really sharp turns. write down how it feels being judgemental.
then add 5 psi to the front and do it again. If you have an empty large parking lot set up some cones or use piles of cothiing to make a slalom course
reduce the front to 25 and add 5 psi to the rear
then run 300 in all 4 and do it again
then add 5 to the front and so on....yo will notice the difference and may be happy around 28 to 30psi. The higher you inflate the tires, the lower the contact patch, the more the tires will squeal in a higher speed turn and that squeal is the tire slipping horizontally on the pavement and is a sign you are approaching the shear traction of the tire at that speed and force. You may get 35 psi in your car for your driving style or you may only get 32 psi.
The lower the contact patch the higher your mpg will increase though.

For a fun car that isnt a commuter I go for more fun and less mpg

 

I vote 30 PSI also. If you have very smooth roads 32 PSI.

 

28-32

 

I run 28 psi in my 255-60R15