How to Convert Bias Ply Tire Sizes to Modern Radials
Your
vehicle is running on bias tires, but you re planning to upgrade them to modern
radial tires. It s a great upgrade that other classic car owners are making,
but you re faced with the challenge of matching the right radial tire size to
the old bias-ply ones.
Of course,
your bias tires contain every bit of information you need to know about your
tires, including the size, but radial tires use different sizing formats from
the old bias ply version. Hence, classic car owners face this challenge when
shopping for modern radial whitewall tires to replace their old tires.
Thankfully,
a simple math work, measurements, and history lesson can help simplify your tire size conversion
assignment.
Why Convert to Radials?
The first
question you want to ask yourself is why you should replace your bias tires
with radial tires, and you have plenty of answers to that.
Upgrading
your tires to radial options isn t only for aesthetics; there s more to their
benefits.
Radial
tires have gained the heart of car enthusiasts with their contributions to
safety and drivability, especially on vehicles used regularly. Bias-ply tires
are linked to a marred driving experience and poor performance at high speeds,
plus they wear faster than their modern radial tire counterparts.
The only
reason you still find bias-ply tires on some cars is to uphold authenticity,
especially at auto shows.
Decoding Bias-Ply Tire Sizes
Before the Sixties
Tire
sizing format in the pre-1960 era was very simple. The format comprised two
numbers separated by a dash. The first number would mean the section width of
the tire (in inches), while the other number after the dash measured the wheel
diameter (also in inches). The aspect ratio (percentage of sidewall height to
width) was typically assumed as approximately 90%, which means there was no
need to be included in the format.
Example:
7.00-15 or 700-15
This
format made it simple for car owners to quickly pick the right tire size for
their cars.
Mid-Sixties to Seventies
Tire
sizing began to get complicated with the introduction of an extended format.
This time, the bias-ply tire sizing format started with a letter indicating the
tire width, followed by the aspect ratio, then a dash before the wheel/rim
width (in inches).
The letter
describing the width ranged from A to L, where A was the narrowest, and L was
the widest. The 78 aspect ratio was the most common among car owners during
this era.
Example:
F78-14.
Decoding Radial Tire Sizing Format
Radial
tire size format is more complex than what we have in bias-ply.
Radial size formatting contains five sets of information. They re:
1. Type
The first
letter hits on the vehicle type. For example, P is for passenger vehicles, and
LT is for light trucks.
2. Width
After the
first letter comes the tire width in millimeters. The width size, in three
digits, is measured from sidewall to sidewall.
3. Aspect Ratio
After the
width, a slash is added before adding the aspect ratio, which is a 2-digit
number.
4. Radial Construction
The number
letter in the arrangement is a compulsory letter R that signifies the tire
features radial construction.
5. Rim Diameter
The first
set of information in radial tire size formatting is the rim diameter. This is
the diameter of the wheel to fit the tire, and it s measured in inches.
Example: P205/75R15
Other information
Many
radial whitewall tire manufacturers add information about the load index and
speed rating to their products. The set of information is separated from the
above information with a space.
The load
index is two numbers that tell the maximum weight the tire can support when
properly inflated, while the speed rating is a letter that indicates the
maximum speed capacity of the tire when used under the recommended conditions.
Example: P205/75R15 91S
How to Convert Sizes
Converting
your traditional bias-ply tire size to radial sizes is simple with these steps:
Take Measurements
The most
important metrics for calculating the right radial-tire size are the section width, overall diameter, and wheel diameter.
Your tire section width is the length between the two sidewalls.
Thankfully,
this set of information is provided on your old number sizing. You only need to
decode the numbers as explained above.
Calculate Key Dimensions
If you own
a bias-ply tire with a 90% aspect ratio, calculate the sidewall height by
multiplying the width by 0.90, and calculate the overall diameter by
multiplying the sidewall height by 2 before adding the rim diameter.
For
example, a 7.00-15 bias-ply tire gives:
Width:
7 inches
Sidewall:
6.3 inches
Diameter:
27.6 inches.
Using a Tire Conversion Chart
While it
is possible to calculate equivalent radial tire sizes using tire dimensions and
aspect ratios, the easiest and most reliable approach is to use a tire
conversion chart. A conversion chart compares common bias-ply tire sizes with
their modern radial equivalents, helping you quickly identify a suitable
replacement.
At Calli
Tire & Wheel LLC, we've created a Bias Ply to Radial Tire Conversion Chart to
help classic car owners find the closest modern radial whitewall tire size.
While conversion charts provide an excellent starting point, factors such as
wheel width, vehicle clearance, and intended use should always be considered
before selecting a replacement tire.
Calli
Whitewall Tires, offer a tire conversion chart for easy matching.
Common Conversions
|
50's
Tire Size |
60's
Tire Size |
70's
Tire Size |
Today's
Modern Tire Size |
|
750-14 |
775-14 |
F78-14 |
|
|
800-14 |
825-14 |
G78-14 |
|
|
850-14 |
855-14 |
H78-14 |
225/70R14 is
not them same height and width, but is the closest available size.
Actual equivalent sizes 225/75R14, 235/70R14, 235/75R14 are not
available by a modern radial tire manufacturer. |
|
560-15
or 600-15 |
165R15 |
||
|
670-15 |
775-15 |
F78-15 |
|
|
710-15 |
815-15 |
G78-15 |
|
|
760-15 |
845-15 |
H78-15 |
|
|
800-15 |
900-15 |
L78-15 |
|
|
600-16 /
650-16 |
215/70R16 is
the size we recommend. It is not the same height and width, but it is
the closest available size in a modern radial tires.
16" bias ply tire radial conversion charts do not exist. Internet
searches give different conversions sizes, but many of them do not correlate
to any actual modern day radial tire size that is currently being made. |
||
Tire size
conversions are intended as general recommendations. Wheel width, suspension
clearance, and the intended use of the vehicle should always be considered when
selecting replacement tires.
Considerations during Conversions
Wheel Compatibility
Most
modern radial tires only fit their original wheels and give issues when
introduced to old rims. Regardless, you want to pay attention to the
recommended wheel width and ensure it matches your old wheels.
Changing
your wheels is more advisable if you re going for wider tires.
Speedometer/Gearing
A diameter
difference of up to 3% is forgivable and can go unnoticed, but larger changes reflect in odometer behavior and tire performance.
Load and Inflation
Experts
advise matching or exceeding the original load capacity, as radials perform
better at higher pressures.
Handling Changes
You ll
likely notice sharper steering and other changes while driving your car. While
this is a positive improvement, you want to be careful when testing your car
after the new upgrade.
Conclusion
Upgrading
your old bias tires to modern radial whitewall tires isn t only about adding
aesthetics to your wheels; you re also investing in a safer, more reliable
driving experience without modifying your vehicle s character. Thankfully,
conversion is easy to achieve.
With
simple formulas and calculations, you can pick the right radial tire size for
your vehicle in minutes.
However,
if you re lost with this conversion, you can talk to a tire expert or supplier.
This is where Calli Whitewall Tires comes in.
Do you
want to replace your bias tires with modern radial options, but don t know how
to go about the size conversion? Perhaps you re on the search for quality
radial whitewall tires to invest in. We have got you covered at Calli Whitewall
Tires.
Contact us today for your tire solutions, or check our list of quality radial whitewall tires of different sizes.
FAQs
Can I combine bias-ply and radial tires on my vehicle?
No. We
don t advise this. It makes sense to replace all your old tires at once.
Driving with different tire constructions is linked to inconsistent handling
and high safety risks.
How close does the radial diameter need to be to the original bias-ply?
Always
stay within 3% difference to maintain speedometer accuracy and gearing.
Do I need new wheels for radial tires?
Are whitewall tires a good option?
Yes.
Whitewall tires offer improved performance of radial tires while beautifying
your wheels.
Where can I find reliable conversion information?
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