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Any difference in recommended pressures front and rear?

You have bias ply or radial?

A 6.5 x 20 would be a bias ply tire. I'm not aware of what pressures Packard might have recommended.

One approach would be to get a recommendation from the tire manufacturer.

Another approach would be to compare the vehicle weight including gas, oil, water and a typical passenger and baggage load to the maximum weight carrying capacity of the 4 tires, as marked on the tire. As the two weights approach each other, the inflation pressure should approach the maximum pressure needed to bear that load. That information should be molded into the tire sidewall.

As mileage accumulates, check the tread wear pattern for signs of under- or overinflation and adjust pressure accordingly.

Maybe try the chalk test? Never done it on a smaller tire, but on off-road tires I run a band of chalk across the tread and drive the vehicle forward and reverse on a level surface. Look at where the chalk was rubbed off and adjust the tire pressure accordingly. Tire pressure is good when it wears off evenly.

Seems like for the 50's cars, regardless of bias ply or radial, the recommended pressure is about 3/4 of the tire max. Packard recommended 24 for bias plys (typical max 32) and radialites usually run 32-35 (typical max 45).

I suspect if you applied Dave's logic you would arrive at such a result for these cars.

Food for thought.

An example. My 8-ply rated 7.00 x 17 bias ply have a rating of 1560 lbs at 40 psi each for a total of 6240 lbs. Naked car weight is 4640 and with gas, oil, water and 2 passengers is probably about 5300 lbs. Experience has shown that a cold inflation pressure of 36-37 psi is best for this set of circumstances.

The manual for my 1928 526 suggests 40 psi. However, it has 600 x 20 tires. The 1931 manual for the 826 - 833 cars and the 840 - 845 cars says that tires should be kept at 40 psi also. I don't have access to a 1929 manual, but I guess the magic number is 40 psi.

Yes, I remember reading that in the manual (I assume it’s similar for my 443) and thinking it’s really high. I’ve kept mine slightly under that (around 35 psi) as I’m a bit terrified of a blowout. I don’t know just how old the tires on mine are but I cannot seem to find a max psi indication on them.

tire, if it's current, will have a max. load and psig rating on it(req'd).

The tires on my Series 526 are at least twenty years old - so are the tubes. I suspect the weak points are the valve stems for the tubes. They have developed small cracks in the rubber, similar to those in the tires. Like you, I am worried about a blowout, and I am saving for new tires this fall.

I find that, in my car, 35 psi makes the steering heavy, especially at low speeds. At 40 psi, the steering is acceptable, and the ride is still not a bit harsh. I am looking forward to see how the ride and handling will be with new tires this fall.