The 1957 Armstrong-Siddeley Sapphire sits in a distinctly British luxury-saloon tradition, blending formal styling with the kind of road manners expected from an upper-market car of the period. That makes tire choice more than a simple size lookup, because this model spans both 15-inch and 16-inch wheel fitments depending on sub-model. The Sapphire 234 uses 185R15 radial or 640-15 bias-ply sizing, while the Sapphire 236 uses 185R16 radial or 670-16 bias-ply sizing. Available choices also vary by brand, sidewall treatment, and tire series, especially on the 15-inch applications. The right selection depends on confirming the exact sub-model and deciding whether period-style bias-ply appearance or radial road use is the better match.
Which Tire Size Is Right for The 1957 Armstrong-Siddeley Sapphire
The 1957 Armstrong-Siddeley Sapphire uses four primary tire sizes, and the correct one depends first on the exact sub-model and wheel diameter. Sapphire 234 applications center on 15-inch fitments, while Sapphire 236 applications use 16-inch fitments, with both radial and bias-ply choices represented. Before selecting a tire, confirm whether the car is a 234 or 236 and verify the wheel diameter currently on the vehicle.
185R15 is a Euro-metric radial size in which 185 identifies the nominal section width in millimeters, R indicates radial construction, and 15 is the wheel diameter in inches. On the 1957 Armstrong-Siddeley Sapphire, this is a 15-inch radial fitment used with the Sapphire 234. Available choices include blackwall, redline, and narrow whitewall presentations from brands such as Coker Classic, Michelin, Avon, and Universal Vintage Tire across several radial series. This size makes sense when the car has the correct 15-inch wheel setup and the goal is a radial tire in a period-appropriate format.
640-15 is a numeric bias-ply size, with the leading 640 representing an approximate 6.40-inch nominal section width and the 15 identifying a 15-inch rim. For the 1957 Armstrong-Siddeley Sapphire, this is the 15-inch bias-ply fitment associated with the Sapphire 234. Product choices include blackwall, narrow whitewall, and wide whitewall options from BFGoodrich, Firestone, Avon, and Excelsior in several classic bias-ply series. This size is the one to evaluate when preserving a more traditional tire construction and appearance matters on a 15-inch Sapphire 234.
185R16 is a Euro-metric radial size, with 185 denoting nominal section width in millimeters, R marking radial construction, and 16 indicating wheel diameter in inches. On the 1957 Armstrong-Siddeley Sapphire, this radial fitment belongs with the 16-inch Sapphire 236 setup. Choices in this size include blackwall radial tires from Michelin, Avon, and Vredestein across series such as Michelin X Radial, Turbosteel CR6D, and Sprint Classic. This is the size to consider when the car carries 16-inch wheels and the objective is radial construction in a classic metric format.
670-16 is a numeric bias-ply size, with 670 indicating an approximate 6.70-inch nominal section width and 16 identifying the required rim diameter. For the 1957 Armstrong-Siddeley Sapphire, this is the 16-inch bias-ply fitment tied to the Sapphire 236. Available options are blackwall bias-ply tires from Firestone and Avon in series such as Firestone Classic Bias Ply and Turbospeed Mk4. This size is the right place to focus when a 236 retains its 16-inch wheel arrangement and a traditional bias-ply construction is part of the car's intended presentation.
Sub-model fitment matters on the 1957 Armstrong-Siddeley Sapphire because the 234 and 236 do not share the same wheel diameter or primary tire sizes. Identifying the exact sub-model is the first step before comparing bias-ply and radial choices.
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1957 Armstrong-Siddeley Sapphire 234
The 1957 Armstrong-Siddeley Sapphire 234 uses the 15-inch tire set, with 185R15 available in radial construction and 640-15 available in bias-ply form. That means selection comes down to whether the car is staying with a more traditional bias-ply presentation or moving toward a radial in the same wheel diameter.
The 1957 Armstrong-Siddeley Sapphire 236 uses the 16-inch tire set, pairing 185R16 radial sizing with 670-16 bias-ply sizing. Tire choice starts with confirming the 16-inch wheel setup, then deciding whether the car's use and appearance goals point toward radial or bias-ply construction.
Bias-Ply to Radial Tire Conversion for the 1957 Armstrong-Siddeley Sapphire
For the 1957 Armstrong-Siddeley Sapphire, the 640-15 bias-ply size has three radial-equivalent options. Those alternatives are 175R15 as an 80-series metric radial, P175/75R15 as a 75-series P-metric radial, and P185/70R15 as a 70-series P-metric radial. Each follows a different sizing convention and profile approach relative to 640-15. A recommended rim-width range of 5-6 inches applies to these equivalent choices, and wheel compatibility, overall dimensions, and clearance should all be checked before making a change.
Modern Radial Equivalents
175R15
Equivalent 80-series Metric Radial (640-15)
P175/75R15
Equivalent P-metric 75-series Radial (640-15)
P185/70R15
Equivalent P-metric 70-series Radial (640-15)
Important Considerations When Replacing Bias Ply With Radial
Compare overall diameter and section width carefully to ensure proper fit.
Verify recommended rim range (5-6 in) and load needs match your vehicle specifications.
Check fender clearance, steering clearance, and stance before installation.
Consider suspension condition and intended road use when choosing equivalents.
Important Fitment Notes
The tire sizes listed on this page assume factory-standard vehicle configuration. The following modifications may affect tire fitment and are not reflected in this guide:
Suspension changes (lowering, lift kits, spring replacements)
Body alterations (fender modifications, custom bodywork)
If your vehicle has been modified from factory specifications, verify tire clearance before purchase.
Best 1957 Armstrong-Siddeley Sapphire Tires for Sale
Available tires for the 1957 Armstrong-Siddeley Sapphire cover both radial and bias-ply construction across 15-inch and 16-inch fitments. Buyers can compare brands such as Michelin, Avon, Firestone, BFGoodrich, Vredestein, Coker Classic, Universal Vintage Tire, and Excelsior, along with blackwall, whitewall, and redline sidewall treatments where offered.
These Armstrong-Siddeley Sapphire tire questions focus on the key decisions for this exact car: identifying the right sub-model and wheel diameter, understanding bias-ply versus radial choices, and comparing appearance, fitment, and road-use considerations before installation.
Tire Size & Fitment
What tire size fits a 1957 Armstrong-Siddeley Sapphire?
A 1957 Armstrong-Siddeley Sapphire does not use one universal tire size across all versions. The Sapphire 234 uses 185R15 in radial form or 640-15 in bias-ply form, while the Sapphire 236 uses 185R16 in radial form or 670-16 in bias-ply form.
The first job is identifying whether the car is a 234 or 236, because that determines both tire size and wheel diameter. Once that is confirmed, the next decision is whether to keep the traditional bias-ply format or choose the radial option that matches that sub-model's wheel setup.
What size rim does a 1957 Armstrong-Siddeley Sapphire use?
The 1957 Armstrong-Siddeley Sapphire uses either a 15-inch or 16-inch rim, depending on sub-model. Sapphire 234 applications use 15-inch tire sizes, and Sapphire 236 applications use 16-inch tire sizes, so rim diameter is a core part of getting the fitment right.
Checking the actual wheel on the car matters, especially if the vehicle has been changed over the years. A buyer should confirm both diameter and width before choosing between radial and bias-ply options, since the available tires in these sizes do not all share the same rim-width compatibility.
Do all 1957 Armstrong-Siddeley Sapphire sub-models use the same tire size?
No, the 1957 Armstrong-Siddeley Sapphire sub-models do not use the same tire size. The Sapphire 234 is centered on 15-inch fitments with 185R15 radial or 640-15 bias-ply sizing, while the Sapphire 236 uses 185R16 radial or 670-16 bias-ply sizing on 16-inch wheels.
That difference affects more than the sidewall marking because it changes the wheel diameter you need to match. Anyone replacing tires should confirm the exact sub-model and inspect the actual wheels on the car, particularly if it has been restored, modified, or fitted with non-original wheels at some point.
Bias-Ply & Radial Options
Is 640-15 a bias-ply or radial tire for the 1957 Armstrong-Siddeley Sapphire?
640-15 is a bias-ply tire size for the 1957 Armstrong-Siddeley Sapphire, specifically on the Sapphire 234. It is part of the car's 15-inch fitment family, while the radial counterpart in that same wheel diameter is 185R15 rather than another 640-series bias-ply marking.
That matters because a bias-ply tire and a radial tire can create a different visual impression and call for different product comparisons. Anyone moving away from 640-15 should confirm the wheel dimensions and compare the intended replacement against the car's clearance and overall setup instead of assuming any radial marked for 15 inches is suitable.
What radial tire sizes are equivalent to 640-15?
Radial equivalents for 640-15 include 175R15 as an 80-series metric radial, P175/75R15 as a 75-series P-metric radial, and P185/70R15 as a 70-series P-metric radial. These are alternative radial sizing formats associated with the 640-15 bias-ply size rather than additional primary fitments.
The practical difference is that each equivalent follows a different sizing convention and profile approach, so the choice is not only about switching construction. A 5-6 inch recommended rim-width range applies to these equivalents, and wheel width, overall dimensions, and body or steering clearance should all be checked before conversion.
What should I check before converting my 1957 Armstrong-Siddeley Sapphire from bias-ply to radial tires?
Start by confirming the car's exact sub-model and wheel diameter, since a Sapphire 234 uses 15-inch fitments and a Sapphire 236 uses 16-inch fitments. Then compare the intended radial size to the wheel width, the car's available clearance, and the way the vehicle will actually be driven and presented.
Construction changes should also be evaluated in the context of the car's current condition. Tire age, suspension wear, steering components, and any previous wheel changes can influence the result, so it is worth inspecting the full setup carefully before choosing a radial alternative for a bias-ply-equipped Sapphire.
Choosing the Right Tire
What should I consider when choosing a 1957 Armstrong-Siddeley Sapphire tire?
The main considerations are the exact sub-model, the wheel diameter on the car, and whether the goal is bias-ply authenticity in appearance or radial road manners. A Sapphire 234 and Sapphire 236 do not share the same primary tire sizes, so size confirmation comes before brand or sidewall decisions.
After the fitment is narrowed correctly, compare construction, rim-width compatibility, sidewall style, tire series, and load figures where they are stated. A restoration-oriented car may lean toward a traditional bias-ply presentation, while a regularly driven car may lead the owner to focus more closely on the radial options in the proper size.
What sidewall styles are available for 1957 Armstrong-Siddeley Sapphire tires?
Sidewall choices for the 1957 Armstrong-Siddeley Sapphire include blackwall, narrow whitewall, wide whitewall, and redline depending on the size and tire series. The 15-inch applications offer the widest appearance range, while the 16-inch products shown here are blackwall-focused.
That variety lets the tire choice support either a restrained touring look or a more visually traditional restoration style. Sidewall treatment should be selected only after the correct size and construction are confirmed, since the available appearance options differ noticeably between 15-inch and 16-inch fitments and between bias-ply and radial products.
What brands offer tires for the 1957 Armstrong-Siddeley Sapphire?
Tires for the 1957 Armstrong-Siddeley Sapphire are available from Coker Classic, Michelin, Avon, Universal Vintage Tire, BFGoodrich, Firestone, Excelsior, and Vredestein across the applicable 15-inch and 16-inch sizes. Brand availability varies by size, construction type, and sidewall treatment.
The useful comparison is not simply brand name, but how each option aligns with the correct wheel diameter and the kind of car being built or driven. Some sizes offer a broader mix of whitewall or redline styling, while others are more limited and should be judged mainly on fitment, construction, and series character.
Restoration & Appearance
Which tire should I choose for a period-style 1957 Armstrong-Siddeley Sapphire?
For a period-style 1957 Armstrong-Siddeley Sapphire, the starting point is the correct sub-model size in its traditional construction format. That means looking at 640-15 for a Sapphire 234 or 670-16 for a Sapphire 236 when the aim is a bias-ply presentation consistent with the car's era.
Appearance choice still leaves room for variation, especially on the 15-inch applications where blackwall, narrow whitewall, and wide whitewall styles are available. Anyone pursuing exact historical accuracy should compare the car against period photos, marque references, and restoration documentation before settling on a particular sidewall treatment.
How does tire construction affect the appearance of a restored 1957 Armstrong-Siddeley Sapphire?
Tire construction can change the visual character of a restored 1957 Armstrong-Siddeley Sapphire even when the wheel diameter stays the same. A bias-ply tire usually supports a more traditional period presentation, while a radial in 185R15 or 185R16 gives the car a more modernized look within a classic-size format.
That difference becomes more noticeable once sidewall style enters the decision, especially on 15-inch applications with redline and whitewall options. Choosing between bias-ply and radial should reflect the car's restoration direction first, then the expected road use, rather than treating construction as a purely cosmetic decision.
Fitment, Maintenance & Road Use
Can modifications affect tire fitment on a 1957 Armstrong-Siddeley Sapphire?
Yes, modifications can affect tire fitment on a 1957 Armstrong-Siddeley Sapphire. Changes such as aftermarket wheels, altered wheel width, different offset, suspension work, or body and fender modifications can influence which of the correct-size tires will physically work on the car.
Even when the sidewall size looks correct, the actual installation can change if the car no longer carries its original-style wheel setup. A careful check of wheel diameter, width, steering clearance, and fender space is worthwhile before selecting either a bias-ply tire or one of the radial options for the Sapphire.
What should I inspect before driving a 1957 Armstrong-Siddeley Sapphire on new tires?
Before driving a 1957 Armstrong-Siddeley Sapphire on new tires, inspect the wheels for correct diameter and sound condition, confirm inflation pressure, and look over the suspension and steering for wear that could affect how the tires run. Lug hardware and valve components also deserve attention during installation.
A short, careful road test is a good way to confirm that nothing rubs and that the car tracks and steers as expected. Collector cars that sit for long periods benefit from extra attention to tire age, prior air loss, and any signs of cracking, bulges, or damage before regular use resumes.
How should 1957 Armstrong-Siddeley Sapphire tires be maintained?
Tires on a 1957 Armstrong-Siddeley Sapphire should be maintained with regular pressure checks, routine visual inspections, and close attention to aging, especially if the car is driven infrequently. Sidewall cracking, tread damage, bulges, and unexplained air loss are all warning signs that deserve prompt review.
Storage conditions matter on collector vehicles, so keeping the car away from prolonged sunlight, excess heat, moisture, and ozone sources helps preserve the tires between drives. It also helps to inspect the tires before longer outings, since low annual mileage can hide age-related deterioration that is not obvious at a glance.
