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Tire pressure radial vs. bias ply #4501459
08/29/17 02:57 PM
08/29/17 02:57 PM
Joined: Jan 2011
Posts: 1,066
Columbus Nebraska
old1 Offline OP
old1  Offline OP
Joined: Jan 2011
Posts: 1,066
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.


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
Re: Tire pressure radial vs. bias ply [Re: old1] #4501477
08/29/17 03:21 PM
08/29/17 03:21 PM
Joined: May 2006
Posts: 759
Portland, Oregon
Joshua_Skinner Offline
Joshua_Skinner  Offline
Joined: May 2006
Posts: 759
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.


2015 Nissan Frontier S King Cab, 2.5L, 5MT
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Re: Tire pressure radial vs. bias ply [Re: old1] #4501482
08/29/17 03:24 PM
08/29/17 03:24 PM
Joined: Apr 2005
Posts: 600
NE Ohio
knerml Offline
knerml  Offline
Joined: Apr 2005
Posts: 600
NE Ohio


2015 Hyundai Elantra Sport 2.0 GDI Mobil 1 5W20 & OEM/Mann W811/80 Filter
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Re: Tire pressure radial vs. bias ply [Re: old1] #4501486
08/29/17 03:34 PM
08/29/17 03:34 PM
Joined: Mar 2013
Posts: 5,037
The Midwest
skyactiv Offline
skyactiv  Offline
Joined: Mar 2013
Posts: 5,037
The Midwest
I've seen an old semi trailer have a tire pressure sticker stating 85 for bias, 100 for radial.


Wife: 15' Audi A4 quattro 6 speed manual
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Wanted: 2019 Ford Ranger
Re: Tire pressure radial vs. bias ply [Re: old1] #4501538
08/29/17 04:46 PM
08/29/17 04:46 PM
Joined: Feb 2003
Posts: 3,480
Somewhere in the US
CapriRacer Offline
CapriRacer  Offline
Joined: Feb 2003
Posts: 3,480
Somewhere in the US


CapriRacer

Visit my web site: www.BarrysTireTech.com
Re: Tire pressure radial vs. bias ply [Re: old1] #4501685
08/29/17 07:18 PM
08/29/17 07:18 PM
Joined: Sep 2015
Posts: 3,941
Connecticut
69GTX Offline
69GTX  Offline
Joined: Sep 2015
Posts: 3,941
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.


----------------

2001 Lincoln Cont 4.6L DOHC/ 39K mi / QS HM 5w30 / FUG XG2
1999 Camaro SS M6 /19K /Mobil 1 0w40 /Fram UG /GM MTL-ATF
1969 Ply GTX/RRs
Re: Tire pressure radial vs. bias ply [Re: Joshua_Skinner] #4501798
08/29/17 08:35 PM
08/29/17 08:35 PM
Joined: Jul 2008
Posts: 685
Maryland USA
rubberchicken Offline
rubberchicken  Offline
Joined: Jul 2008
Posts: 685
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.

Re: Tire pressure radial vs. bias ply [Re: old1] #4501908
08/29/17 11:43 PM
08/29/17 11:43 PM
Joined: Aug 2017
Posts: 225
Cincinnati, USA
Dave9 Offline
Dave9  Offline
Joined: Aug 2017
Posts: 225
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.

Re: Tire pressure radial vs. bias ply [Re: old1] #4502022
08/30/17 06:23 AM
08/30/17 06:23 AM
Joined: Feb 2003
Posts: 3,480
Somewhere in the US
CapriRacer Offline
CapriRacer  Offline
Joined: Feb 2003
Posts: 3,480
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.


CapriRacer

Visit my web site: www.BarrysTireTech.com
Re: Tire pressure radial vs. bias ply [Re: old1] #4502079
08/30/17 07:34 AM
08/30/17 07:34 AM
Joined: Jan 2011
Posts: 1,066
Columbus Nebraska
old1 Offline OP
old1  Offline OP
Joined: Jan 2011
Posts: 1,066
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.

Last edited by old1; 08/30/17 07:37 AM.

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
Re: Tire pressure radial vs. bias ply [Re: old1] #4502347
08/30/17 01:24 PM
08/30/17 01:24 PM
Joined: Feb 2003
Posts: 3,480
Somewhere in the US
CapriRacer Offline
CapriRacer  Offline
Joined: Feb 2003
Posts: 3,480
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.

Last edited by CapriRacer; 08/30/17 01:28 PM.

CapriRacer

Visit my web site: www.BarrysTireTech.com
Re: Tire pressure radial vs. bias ply [Re: old1] #4502460
08/30/17 02:51 PM
08/30/17 02:51 PM
Joined: Dec 2012
Posts: 18
Holland
jadatis Offline
jadatis  Offline
Joined: Dec 2012
Posts: 18
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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