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While the "MAX" listed on the tire itself may not be that which the manufacturer suggests, it will be a "safe" limit, a guideline, and with higher pressure it may reduce the potential for hydroplaning, but will be a more firm ride than the likely lower pressure in the owners' manual, and on either the doorpost, gas flap, or glovebox door where the manufacturer may have initially placed an inflation sticker. 

 

You might also "Google" for tire pressure with your year, make, and model - surprising how much information is out there

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Measure the width of the tread. Calculate the area in square inches of a circle that size. Divide that into the weight resting on the tire. Put in that many pounds per square inch. My estimate,  30 pounds.

 

You can go 10% or 15% lower for a soft boulevard ride, or 10% or 15% higher for high speeds above 40MPH.

Edited by Rusty_OToole (see edit history)
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Further research reveals that no Chrysler came with 650X20 tires at least none that I can find.Closest is the 1928 Imperial six that came with 6.75X20. They weighed from 3870 to 4300 pounds. OP does not state year, model, body style or any other relevant information so I will guess some things. Let's take a typical touring car that weighs 4000 pounds with 4 passengers and baggage totaling 800 pounds, giving a GVW of 4800 pounds.

 

Assuming weight distribution of 40/60 front/rear that works out to 28 pounds in the front tires and 43 in the back. With 2 passengers, 40 pounds. For a soft ride you could drop it to 25 front/ 35 rear or for high speed travel, increase to 30 to 32 front and 44 rear, more if the car is heavily loaded.

 

That is the best estimate I can do on the limited information given. This is purely theoretical. Feedback from experienced drivers of late twenties luxury cars would be appreciated.

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8 hours ago, Mark Shaw said:

Just read the max cold pressure on the tire itself.

 

Yes. Conventional wisdom is evolving to this, rather than the lower "boulevard cruiser" manufacturers specs. And if you do read the fine print on the sidewall, you will see that this is not the max SAFE pressure, but the max LOAD BEARING pressure. Lots of old car drivers, myself included, OVER inflate, particularly on the fronts, by about 10%. A control thing which SMOOTH roads in good condition allow. I emphasize "smooth" for the reason Marty mentions above. Too high pressure, too firm the ride, can harm your tires and/or suspension components. Marty drives a greater variety of old cars farther and more frequently than most of the rest of us do, sadly, myself included at this late stage of life

 

Tire pressure is really, as Rusty' alludes to, more of a science than even owners or manufacturers data will provide. I must point out in Rusty's recent posting, that 40/60 weight distribution is almost unheard of in most every realm of automotive production. Old live axle cars have the massive engines and transmissions set back in the frame. They actually have pretty good balance. Usually do have a bit more weight forward. As an aside, such forward weight distribution also increases aerodynamic stability. Ideally, aerodynamics at speed wants the center of gravity ahead of the center of pressure. Gabriel Voisin became painfully aware of this well-known aviation necessity's relevance to automotive stability after the loss of his son in a high speed automobile accident. Though I don't know, I speculate that Bernd Rosemeyer lost his life in the aft-heavy Auto Union land speed record attempt due to insufficient forward ballast. Smells like it to me from the side gust circumstances under the bridge that sad day. Uh, I really do have a tendency to run on with marginally relevant trivia. Anyway, regarding weight distribution in tire pressure selection, please re-read the last 2 sentences in the first paragraph. Speed, and road (or degree of lack thereof - eg. sand, mud, rock, ruts, washboard, gravel and so on), condition, variable traction surface, tread design, etc. all play into tire pressure selection. Driver experience is also a factor. I was an experienced long distance adventure driver, and son of same. I took it to another level. Lots of off road, and even no road components to my antics. Some foreign long distance speed record stuff. Driver of choice for some joint U.S./British Embassy "joy ride" jaunts. Please don't get in my way. I had Read books by the racers, and the adventurers who had gone before me. Then went out to practice and perfect their techniques. We refer to tires as the "tunable part of the suspension". And there is no tire more tunable than radials. Much more operating range. Some can go from around 12 psi (soft sand - remember this trick if you like beach driving. We would drive beaches at minimum ROAD pressure until we got stuck. Then air down to 15-18 psi and turn back to from whence we came. Confident of safe return), all the way up to maybe 50 psi for cruise speed. Same tire, different conditions. Make sure to inflate to road pressure before hitting  the pavement again after the day(s) on the beach.

 

Capndan, I don't know the facts surrounding your present 6.50x20's. Are they good road tires, or static/show tires ? Please enter Samson in the search box. About the 3rd or 4th link down, a posting by me, will get you to a topic including tires on Derek's (Thriller on the forum), 1929 Buick. He found the finest 20" tire in the world for guys lucky enough to be running 6.50s, 6.75s, or 7.00s. If you are running show rubber, but want to drive your car anywhere,  shed those flimsy skins and get a set of those 14 ply rated Samson 7.00x20 truck radials. 

 

A few last words  : NEVER intentionally be out solo in the wilds without a spare tire. That is why we always go OUT with TWO spares. THEN, if you get a flat, you can head home at that point still having a spare. That was an old technique back in the days when roads and tires were not what they are today. If you are still strong enough to travel to, and rent a car to explore some exotic 3rd world paradise, carefully examine all tires, including the spare. Make SURE the spare actually fits. If you are adventurous and plan on putting some real miles on the rental, buy another mounted spare. Been there, done that. A little like going back to the old days and techniques.

 

                          What do you think capndan ?     Much shorter legged these days,   -   Cadillac Carl 

 

Edited by C Carl
Correct an "oops" (see edit history)
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  • 9 months later...

I am bringing this back in order to add to the current topic about radial tires for a 1928 Studebaker President. Way too much content to transcribe, and I have no idea how to cut and paste (or whatever it is that I would have to do in order to transfer anything from place to place here). Also, I have Resurrected Derek Thille's (Thriller on the forum), topic 1929 Came Home Today,  down in Buick - Pre War - Technical which has very good info needing a status update. High quality truck radials are a VERY different product from puny 36 psi "thin skins".     -    CC 

Edited by C Carl (see edit history)
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I suspect a load range E (10 ply rating) tire would be as strong. Actually a LR E ST tire might not be bad: are very strong and not for over 65 mph may not be a problem. What size are we talking about ? 175-185 x 20 ?

 

Funny thing about Load Ratings, they derive from the max allowable inflation pressure (LR E = 80ps maxi) and the pressure times the load volume times a fudge factor equals the pounds  rating.

Edited by padgett (see edit history)
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Most Chryslers of the late 20's run 40psi in the front and 35psi at the rear.

 If the car has wood wheels with split rims, then make sure you do not run with too little pressure, as it could cause you to blow a tire

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