No one in their right mind ever ran 22 psi. Low pressures were set to deliver soft ride with minimal load (3 pass?, empty trunk?). Manuals typically said add 4 psi for full load (5 pass? + suitcases). Also add 4psi for high speed (65 mph and above in those days). But in the real world, we (Dad was a tire dealer 1955-84) set pretty much everything at 30 psi unless owner specified otherwise or 6 ply (trucks 35-45 psi and some wagons might go 36 psi).
1957-1962 8.50-14 4 ply load ratings - 1205 lb @22 psi, 1265 lb
@24 psi
1957-1962 7.60-15 4 ply load ratings - 1190 lb @22 psi, 1255 lb @24 psi, 1310 lb @26 psi
1968 8.50-14 4 ply rated (2 ply) load ratings - 1420 lb @22 psi, 1480@24, 1550@26, 1610@28, 1670@30, 1740@32
1968 8.55-14 4 ply rated load ratings - 1430@22psi, 1510@24, 1589@26, 1640@28, 1710@30, 1770@32
1968 8.45-15 (repl 7.60-15) load ratings - 1410@22, 1480@24, 1550@26, 1620@28, 1680@30, 1740@32
1986 P225/75R14 (8.50-14) load ratings - 1433@22, 1499@24,
1554@26, 1609@28, 1664@30, 1720@32, 1797@35 (max recommended
pressure for "standard" - not "extra" - load rating)
1986 P225/75R15 (7.60-15) load ratings - 1499@22, 1565@24, 1631@26, 1687@28, 1742@30, 1797@32, 1874@35 (max recommended pressure for "standard" - not "extra" - load rating)
2009 P215/75R14 (load index 98) - 1433@26, 15221@29, 1587@32, 1664@35
I list the P215/75R14 knowing it is the old 8.00-14 equivalent
which is theoretically undersize for 300s, but it is the last
biggest 14 currently in mass (not specialty) production readily
available at "reasonable" prices and it would have adequate load
capacity despite being smaller diameter (lower ride height).
2009 P225/75R14 (load index 101) - 1554@26, 1642@29, 1720@32, 1797@35
2009 P225/75R15 (load index 102) - 1631@26, 1720@29, 1797@32, 1874@35
2009 P235/75R15 (load index 105) - 1753@26, 1852@29, 1940@32, 2028@35
I list the P235/75R15 (old 8.00-15) here because it is a better
match for the overall diameter of the 7.60-15 and it is more
readily available in mass produced "non-specialty" as well as
"specialty" tires in more varieties than the P225/75R15.
Personally I prefer the mass-produced tires from a name I
recognize as they tend to be more modern design and manufacture
and have tighter tolerances and better quality control than most
"specialty" tires which are typically produced in small batches,
frequently using older technology. Also there are the issues of
cost and availability.
NOTE: These are all COLD INFLATION PRESSURES.
The most recent tables I have are 2009 and do not reflect higher
pressures than 35 psi or lower recommended pressures than 26 psi.
But most current "standard" load radials are labeled max load xxxx
at 44 psi. Personally I would go 32-35 psi in a current vintage
bias or radial on a vintage car regardless of load. Clearly that
would be more than sufficient for the load (assuming you have the
correct size tire), more than that would give you a pretty rough
ride, and less than that would give you more "radial squat".
Hope this helps - Best, Ed
--I've been running radials on old mopars since I first switched over on my 1964 300 back in the late 70s. I pretty much run them at 32 psi on everything. My 300F has American Classics and Coker Classics before that and it seems to be the best pressure. Certainly couldn't run the 22 psi that the original bias tires were run at.
Nick
--On Fri, Jul 26, 2024 at 9:17 AM 'James Douglas' via Chrysler 300 Club International <chrysler-300-club-international@xxxxxxxxxxxxxxxx> wrote:
--I am sure this will open up a pandoras box…
I would like to know how people are dealing with the subject of tire pressures with modern radials on those older Chryslers?
The Service Manual pressures for bias ply are obviously way too low. So, how are people arriving at a “proper” tire pressure for optimum use and tread life?
It would be nice if the tire companies put on their websites the load-pressure graphs they no doubt have. But of course, they will not.
If one runs at the max pressure on the tires and the car is substantially less in weight, then the tires will not perform as well or wear as well as the should. Same if they are under inflated.
So, I would like to hear how people are dealing with this issue.
Thanks, James
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