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#4501459 - 08/29/17 03:57 PM Tire pressure radial vs. bias ply
old1 Online   content


Registered: 01/04/11
Posts: 1001
Loc: Columbus Nebraska
Just curious if there is any formula to follow when using modern radial tires on a classic car originally equipped with bias ply tires? I have the original owners manual, and it says 24 to 28 lbs depending on speed and load.
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1965 Mustang 200 c/4 10w30 QS defy
1964 Ford Ranchero 302 c/4 15/40 Delo
1929 ford model A No filters, and 15/40 oil

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#4501477 - 08/29/17 04:21 PM Re: Tire pressure radial vs. bias ply [Re: old1]
Joshua_Skinner Offline


Registered: 05/08/06
Posts: 730
Loc: Portland, Oregon
Radial tire construction is less affected by tire pressure than bias ply. For example too much pressure will wear the center out of a bias tire, but rarely on a radial. I run my '63 Studebaker around 30 psi and it's wearing 215/65R15 tires on 6" wide chrome smoothies.
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#4501482 - 08/29/17 04:24 PM Re: Tire pressure radial vs. bias ply [Re: old1]
knerml Offline


Registered: 04/05/05
Posts: 480
Loc: NE Ohio
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#4501486 - 08/29/17 04:34 PM Re: Tire pressure radial vs. bias ply [Re: old1]
skyactiv Offline


Registered: 03/02/13
Posts: 4396
Loc: The Midwest
I've seen an old semi trailer have a tire pressure sticker stating 85 for bias, 100 for radial.
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#4501538 - 08/29/17 05:46 PM Re: Tire pressure radial vs. bias ply [Re: old1]
CapriRacer Offline


Registered: 02/22/03
Posts: 3369
Loc: Somewhere in the US
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#4501685 - 08/29/17 08:18 PM Re: Tire pressure radial vs. bias ply [Re: old1]
69GTX Offline


Registered: 09/23/15
Posts: 3815
Loc: Connecticut
I had classic muscle cars for many years and drove most of them on radial tires at modern day pressures (30-34 psi). Seems to me it would have more to do with the tire itself (radial or BP) than the car. When I had bias ply tires on several of those cars for show points, I kept them at the same 30-34 psi pressures. Only one set of bias ply tires wore a bit oddly, with the outer blocks wearing faster than the inside. That was only after 1,000-2,000 miles or so. That suggested under-inflation, which was not the case. The ride on those bias ply tires was marginal..and they always felt hard. Every little groove in the highway would grab them and toss the car around. Maybe I should have kept them at 26-28 psi. In looking at some Mopar tire stickers for 1968-1970 B body, pressures ranged from 24-30 psi depending on # of passengers.
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#4501798 - 08/29/17 09:35 PM Re: Tire pressure radial vs. bias ply [Re: Joshua_Skinner]
rubberchicken Offline


Registered: 07/07/08
Posts: 399
Loc: Maryland USA
Originally Posted By: Joshua_Skinner
Radial tire construction is less affected by tire pressure than bias ply. For example too much pressure will wear the center out of a bias tire, but rarely on a radial. I run my '63 Studebaker around 30 psi and it's wearing 215/65R15 tires on 6" wide chrome smoothies.


I had a Vette with Goodyear Gatorbacks (radials), and had a big problem wearing the center of the rear tires. It turned out that my high-dollar tire pressure gauge was off, leading to the tires being 2 lbs too high- only 2 lbs. It probably wasted about 1/3 of the potential tire life, so it was a costly error.

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#4501908 - 08/30/17 12:43 AM Re: Tire pressure radial vs. bias ply [Re: old1]
Dave9 Offline


Registered: 08/28/17
Posts: 176
Loc: Cincinnati, USA
Take a white crayon and draw a line across the tread. Inflate the tire to its max sidewall rated PSI. Drive it in a straight line a few dozen yards. Observe crayon, let some air out in increments until the straight line test starts to scrub crayon off the outer tread area, or just before that area if it has more of a sloped shoulder molding.

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#4502022 - 08/30/17 07:23 AM Re: Tire pressure radial vs. bias ply [Re: old1]
CapriRacer Offline


Registered: 02/22/03
Posts: 3369
Loc: Somewhere in the US
Just so everyone understands:

OP lists 3 vehicles in his signature. Of interest is a 1965 Ford Mustang and a 1964 Ford Ranchero.

According to Tire Guides, a 1965 Mustang came with either 6.50-13's or 6.95-14's. These have 88% and 82% aspect ratios, respectively. Obviously they do not fit into the current sizing system where 80% is the highest aspect ratio.

The second question is: What tire size is he using now? Did he put on aftermarket wheels and a much lower aspect ratio?

Now the Ranchero isn't even listed in Tire Guides, so I can't tell what the vehicle came with, but in that era, something similar was used.

So we need to have some information that is missing.

Oh, and the *Chalk test*? It's only valid IF the tire has a good profile - and many tires don't. You could wind up with way too low a pressure and risk a tire failure.
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#4502079 - 08/30/17 08:34 AM Re: Tire pressure radial vs. bias ply [Re: old1]
old1 Online   content


Registered: 01/04/11
Posts: 1001
Loc: Columbus Nebraska
The mustang has the original 14" wheels, and 195-75-14 tires. The Ranchero has some steel 15" ford wheels that are wider, and 225-60-15 tires. The mustang is the one of most concern. I have been running around 30 psi, and seems to be ok, I get good gas mileage with the 200 6 (average 23mpg). but was wondering if lower pressure would make the ride smoother, as lots of the roads around here are not very good.


Edited by old1 (08/30/17 08:37 AM)
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2017 Nissan Frontier 4.0 5w30
1965 Mustang 200 c/4 10w30 QS defy
1964 Ford Ranchero 302 c/4 15/40 Delo
1929 ford model A No filters, and 15/40 oil

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#4502347 - 08/30/17 02:24 PM Re: Tire pressure radial vs. bias ply [Re: old1]
CapriRacer Offline


Registered: 02/22/03
Posts: 3369
Loc: Somewhere in the US
old1, thanks for getting back to us.

A 6.95-14 at 24 psi had a load carrying capacity of 1160#. To get the same load carrying capacity, a P195/75R14 needs to use :drumroll: 24 psi!

But the lowest pressure I think is suitable for current P metric tires is 26 psi. Further, in that era, it was common for the OEM's to specify tires that were barely adequate. I've documented 3 times since that period where the OEM's upped the tire size (for various reasons).

That makes 30 psi seem like a good value.

Also, the smallest wheel a P195/75R14 fits on a 5" - which is what Ford supplied.

But what about the Ranchero? Nowadays, I think the vehicle would be reinforced so it could carry a reasonable amount of cargo - but back then????? It was built on a Falcon chassis and Falcon wagons used 7.00-13's inflated to 24/28 psi.

A 7.00-13 has a load carrying capacity of 1080# at 24 psi and 1180# at 28 psi. To get the same load carrying capacity in a P225/60R15 takes ..... ah ....... well, the chart doesn't go that low and even extrapolating, I get a ridiculously low value.


Edited by CapriRacer (08/30/17 02:28 PM)
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#4502460 - 08/30/17 03:51 PM Re: Tire pressure radial vs. bias ply [Re: old1]
jadatis Offline


Registered: 12/20/12
Posts: 15
Loc: Holland
If you know weights on axles , or better on seperate tires, I am able to calculate a save lowest pressure .
so its not important what the old advices where, yust give weights and speed used as max of vehicle, and of tires :1 maxload or loadindex :2 kind of tire ( think P tire in standard load or XL/reinforced/Extraload) and 3: speedcode of tire ( letter Q and above most used for radial tires ).

For old cars also often oversised tires where used if normal persons-car, for Trucks often yust enaugh maxload to carry the max allowed axle weights.

So weigh the car once , loaded as you mostly use it, and it will count for years.

The calculation of needed tirepressure for Bias Ply tires is what the tires organizations agreed a formula for in 1928, and for radial tires they chanched it about 1970 , different in America and Europe.
Nowadays the European formula for all kind of tires , is used in America since 2006 but only for P-tires ( in SL and XL//).
If you would have a Bias Ply and radial tire with same maximum load and pressure needed for that ( called reference-pressure in offical European Formula , I once got hold of) It would give lower pressure for the same load on tire, for bias ply then for radial ply.

The 26 psi given by capry racer as minimum for P tires , is in European system 21 psi/1.5 bar.

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