Vintage Motorcycle Tire Sizes Decoded: How to Read Every Format on a Classic Bike

Pull a vintage motorcycle out of long-term storage and one of the first puzzles is the rubber. Your owner's manual calls for 3.50-19 front and 4.00-18 rear. Your dealer catalog from 1972 lists those sizes. But the tire shop's database wants 100/90-19 and 110/90-18. The Avon catalog lists the same tire as MJ90-19 and MN90-18. The Bridgestone equivalent is 90/90-19 and 110/80-18. Same wheel, same bike, four different sets of numbers, and none of them match the math you'd expect.

This isn't sloppy labeling. It's the result of three sizing systems being grafted onto a fourth over six decades of motorcycle history, each one trying to describe the same tire in a different way. If you know what each format encodes, you can shop confidently across catalogs, decades, and brands. If you don't, you end up running a tire that's the wrong overall diameter, the wrong profile for your rim width, or the wrong speed rating for your bike. This guide decodes every format you'll encounter on a vintage bike, with conversion references and the practical fitment notes that the catalogs leave out.

The Four Sizing Systems You'll Find on Vintage Bikes

Four formats are in active use on vintage motorcycle tires. The order they appeared in roughly tracks the eras of the bikes you'll find them on.

System	Example	What It Encodes	Era
Standard Inch	4.00-18	Width in inches, rim diameter in inches, ~100% aspect ratio assumed	Pre-1970s
Low-Profile Inch	4.25/85-18	Width in inches, aspect ratio (82% default or 85% explicit), rim diameter	1970s
Alphanumeric (Alpha)	MT90-16	M = motorcycle, letter = width code, aspect ratio, rim diameter	Late 1960s through today on some Harleys
Metric	130/90-16	Width in millimeters, aspect ratio percentage, rim diameter in inches	Mid-1980s onward, modern standard

A vintage bike that left the factory with inch-marked rubber may now wear alpha or metric replacements, because the original size no longer rolls off any modern production line. Knowing how to translate between systems is what makes that swap work.

Standard Inch System (3.50-19, 4.00-18)

The oldest format you'll see on a classic motorcycle is two numbers separated by a dash. The first is the section width in inches. The second is the rim diameter in inches. Aspect ratio isn't called out, because in this system it's assumed to be approximately 100% — meaning the sidewall height is roughly equal to the width. The tire profile is essentially square in cross-section.

A 3.50-19 reads as 3.5 inches wide with about 3.5 inches of sidewall height, mounted on a 19-inch rim. A 4.00-18 is 4 inches wide with 4 inches of sidewall, on an 18-inch rim. A 5.00-16 — the dominant size on Big Twin Harleys from the 1930s through the early 1970s — is 5 inches by 5 inches on a 16-inch wheel.

This format dominated vintage British twins (BSA, Triumph, Norton), early Honda CBs, vintage Harleys, and most pre-1970s European bikes. Some inch-marked tires also include a speed rating letter inside the size string, like 3.50H19, where the H falls between the width and the rim diameter. The H is the speed rating, not part of the dimension.

The standard inch system is being phased out of new tire production. You can still buy genuine inch-sized tires from specialty makers — Coker, Firestone, Avon, Excelsior, Dunlop K70 — but for many sizes, your only path forward is finding the metric equivalent and verifying it fits your rim and your fender.

Low-Profile Inch System (4.25-18, 4.25/85-18)

By the early 1970s, motorcycle handling expectations had moved past square-profile tires. Manufacturers wanted shorter sidewalls for better cornering and steering response. The low-profile inch system kept the inch-based width and rim measurements but added a non-100% aspect ratio.

The catch is that the aspect ratio is often invisible. A tire marked 4.25-18 with no second number is assumed to be 82% aspect ratio by default — meaning the sidewall height is 82% of 4.25 inches, or about 3.49 inches. If the manufacturer wanted to specify an 85% profile, they'd write it explicitly: 4.25/85-18, with sidewall height of about 3.61 inches.

This format had a short production window and is now genuinely rare on new tires. You'll mostly see it in vintage reproduction stock and the occasional off-road tire. If you have a bike from the mid-to-late '70s wearing low-profile inch rubber, expect to convert to metric for any modern replacement.

Alphanumeric Alpha System (MT90-16, MH90-21)

The alpha system was the bridge between inch and metric. It appeared in the late 1960s as a way to standardize tire sizing across an industry that was starting to globalize, and it's still used on some Harley-Davidson touring bikes and replacement tires today.

An alpha size like MT90B16 breaks down as four pieces:

• M — motorcycle. This always comes first and never changes. It tells you the tire is built for motorcycle use rather than passenger car or trailer service.
• T — width code. The second letter identifies the section width using a coded scale. The most common width codes are MH (80mm), MJ (90mm), MM (100mm), MN (110mm), MP (110mm), MR (120mm), MT (130mm), MU (140mm), and MV (150mm).
• 90 — aspect ratio percentage. Sidewall height as a percent of width, just like the metric system.
• B — construction. B means bias-belted. R means radial. A dash or no letter means standard bias-ply.
• 16 — rim diameter in inches.

So MT90B16 reads as a motorcycle tire, 130mm wide, with sidewall height 90% of 130mm (about 117mm), bias-belted construction, on a 16-inch rim.

The alpha system tops out around MV (150mm). Anything wider was developed after the system was already being replaced by metric, so wider tires use metric labeling exclusively. If you're shopping for a replacement on a vintage Harley touring bike, alpha-marked options will frequently appear next to metric equivalents in the same size — they're describing the same tire.

Metric System (110/90-18, 130/90-16)

The metric system became the global standard by the mid-1980s and remains the default format on virtually every motorcycle tire sold today. Three pieces of data, separated cleanly:

• 110 — section width in millimeters, measured from sidewall to sidewall when the tire is mounted and inflated.
• 90 — aspect ratio. Sidewall height as a percentage of width. A 110/90 has sidewall height of 99mm (90% of 110).
• 18 — rim diameter in inches. The metric system kept inch-based rim sizing because every wheel in the world is still measured in inches.

You'll often see additional codes in the metric size string. A construction letter (R for radial, B for bias-belted) typically falls between the aspect ratio and the rim diameter: 120/70R17 is a 120mm radial. A speed rating letter may appear immediately before the construction code (120/70ZR17 means Z-rated radial). After the size, you may see a load index and speed rating combination like (58W) — that's the load capacity number plus the speed rating.

For vintage motorcycle owners, the metric format is what you'll see on most modern replacement tires designed to fit classic bikes. Continental's ContiClassicAttack, Heidenau K34, Avon Roadrider MkII, and the bias-look radials from Autobahn all carry metric sizing even when designed specifically for vintage applications.

Conversion Reference: Inch to Metric to Alpha

The chart below cross-references the most common vintage motorcycle tire sizes across all three modern formats. Use it as a starting point — verify final fitment against your rim width spec and clearance before buying.

Inch Size	Metric Equivalent	Alpha Equivalent	Typical Use
3.00-21	80/90-21 or 90/90-21	MH90-21	Vintage front, narrow rim
3.25-19	90/90-19 or 100/90-19	MJ90-19	British twins, early Honda CB front
3.50-19	90/90-19 or 100/90-19	MM90-19	Triumph, BSA, vintage Honda front
3.50-18	90/90-18 or 100/90-18	MM90-18	Vintage Brit and Japanese mid-size
4.00-18	110/90-18	MN90-18	Vintage rear, '60s-'70s standards
4.25-18	110/90-18 or 120/90-18	MR90-18	Late '70s low-profile rear
4.50-18	120/90-18	MR90-18	Vintage rear, larger displacement
5.00-16	130/90-16	MT90-16	Vintage Harley Big Twin
5.10-16	130/90-16	MT90-16	Vintage Harley Big Twin variant

Two things to note about this chart. First, where two metric options are listed for one inch size, the lower aspect ratio (90/90 vs 100/90) gives you a closer match to the original overall diameter, while the wider option offers more rubber on the rim and a slightly modernized look. Second, alpha equivalents are exact translations of the metric width — MN90-18 always means 110/90-18, period.

Why the Math Doesn't Always Work

Here's where the cross-reference chart starts arguing with the calculator. A 4.00-18 should convert to about 101.6mm wide (4 inches × 25.4 mm/inch). But every modern fitment guide tells you to replace a 4.00-18 with a 110/90-18, not a 100/90-18. The 110 is roughly 8mm wider than the math says it should be. What's going on?

Two answers. First, vintage tire dimensions were always nominal. Old 4.00-18 tires were spec'd to a target but actually measured anywhere from 4.25 to 4.6 inches wide depending on the manufacturer, the production batch, and what rim they were mounted on. The "4.00" was a label, not a precise measurement. The Bridgestone Spitfire S11 — the standard mid-size rear tire of the 1980s — measured 4.6 inches wide in a 110/90-18, which is exactly what those old "4.00" tires actually were in the real world.

Second, modern tire profiles are slightly different than vintage profiles, even at the same nominal dimensions. Modern compounds and casing constructions sit a touch differently on the rim, so a 110/90-18 mounted on a vintage 1.85-inch rim will measure pretty close to what an original 4.00-18 measured on the same rim back in 1972.

The practical conclusion is that you trust the cross-reference charts published by tire manufacturers (Bridgestone, Avon, and Continental all maintain them) over your own math. If a chart says 110/90-18 replaces a 4.00-18, that's the tire that will actually fit and feel right, regardless of what the conversion factor says. If you change overall diameter by more than a few percent, you'll throw off your speedometer accuracy and potentially your gearing feel — keep mounted OD within roughly 3% of the original to avoid those problems.

Speed Rating, Load Index, and Construction Codes

Beyond the size itself, vintage motorcycle tires carry additional codes that matter for both safety and fitment. The most important are speed rating, load index, and construction.

Speed rating is a single letter that indicates the maximum sustained speed the tire is designed to handle. The most common ratings on vintage motorcycle tires are J (62 mph / 100 km/h), S (112 mph / 180 km/h), H (130 mph / 210 km/h), V (149 mph / 240 km/h), and Z (149+ mph / 240+ km/h). On inch-marked tires, the speed rating is often embedded in the size string itself: 3.50H19. On alpha and metric tires, it appears just before the rim diameter or as part of a load/speed index after the size.

Load index is a number that codes the tire's maximum carrying capacity. You'll see it on modern replacement tires for vintage bikes as part of a service description like (58W), where 58 is the load index and W is the speed rating. Vintage tires often used a ply rating (4PR, 6PR) in place of a load index. The two systems aren't directly equivalent, but the spec from your motorcycle's owner's manual tells you what minimum load capacity you need.

Construction codes are a single letter indicating how the tire is built. R is radial. B is bias-belted (cross-ply with stabilizing belts under the tread). A dash or no letter means standard bias-ply. Most vintage motorcycles were designed around bias-ply construction, and many vintage owners prefer the ride feel and sidewall stability that bias-plies provide. Bias-belted hybrids and "bias-look radials" — radials with a tread pattern designed to look period-correct — are increasingly common as a compromise between authentic appearance and modern grip.

TT and TL markings tell you whether the tire is built for tube-type or tubeless service. Vintage spoked rims almost always require tubes, regardless of what the tire is rated for. Mounting a tubeless-rated tire on a tube-type rim still requires a tube. The reverse — mounting a tube-type tire tubeless — is not safe and not recommended.

M/C designation simply means the tire is built for motorcycle use. Some sidewalls include this marking explicitly; others omit it because the size format itself already implies motorcycle service.

Common Vintage Sizes by Bike Type and Era

If you don't have your owner's manual or factory specs handy, the table below covers the typical OEM sizing for the most common vintage motorcycle categories. These are starting points — always verify against your specific year and model before buying.

Bike Category	Typical Front	Typical Rear
Pre-WWII American (Indian, vintage Harley)	4.00-18 or 4.50-18	4.00-18 or 4.50-18
Vintage Harley Big Twin (1936-1973)	5.00-16	5.00-16
British twins (Triumph, BSA, Norton, 1950s-1960s)	3.25-19 or 3.50-19	3.50-19 or 4.00-18
Vintage Honda CB (1960s-early 1970s)	3.25-19	3.50-18 or 4.00-18
Vintage Japanese mid-size (mid 1970s)	3.50-19 or 100/90-19	4.00-18 or 110/90-18
Late Shovelhead and FX models (1976-1984)	MT90-19 (130/90-19)	MT90-16 (130/90-16)

What to Do When Your Original Size Isn't Made Anymore

Plenty of vintage tire sizes are still produced new — 5.00-16, 4.00-18, 3.50-19, and most of the British and American standards remain available from Coker, Firestone, Avon, Indian, and a handful of specialty brands. But some sizes have genuinely fallen out of production, and the conversion process gets more involved.

The first principle is to maintain the original mounted overall diameter as closely as possible. Changes to OD beyond about 3% will throw off your speedometer accuracy, alter your effective gearing, and can mess with your bike's handling balance. The cross-reference chart above gives you a starting point, but you should also calculate the OD of any candidate replacement against the original spec.

The second principle is to verify the replacement size fits your rim width. Vintage spoked rims are narrow by modern standards — typically 1.60, 1.85, or 2.15 inches wide. A modern metric tire designed for a wider rim will sit with a flattened profile on a narrow vintage rim, with the contact patch crowning up more than the tire was designed for. Most tire manufacturers publish rim width charts that specify the acceptable range for each size; stay inside that range and the profile will look right.

Third, check fender, chain, and fork clearance with the replacement size before you commit. A wider tire that technically fits the rim may rub a fender stay at full compression, contact the chain on the drive side, or bind on the swingarm at full lock. Test these clearances with the bike on the ground, fully loaded, with the steering at full lock both ways.

Performance Plus Tire stocks vintage motorcycle tires across all the major formats — bias-ply originals like the Coker Classic Motorcycle 5.00-16, the Avon SpeedMaster MkII 4.00-18, the Firestone Zig Zag M/C, and the Indian INDIAN Script Motorcycle in both 5.00-16 and 4.00-18, plus modern bias-look radials like the Autobahn Bias Look Radial 5.00-16 for owners who want vintage appearance with modern construction. The full Vintage Motorcycle Tires catalog is the place to start when you know your size, and the Antique Tires section covers pre-1940s applications. If you're working through a broader vintage build that includes the rest of the bike's rubber, our vintage motorcycle tires safety guide covers the construction and aging issues that affect all vintage rubber, and the collector's guide to authentic period rubber walks through the period-correct options brand by brand.

For deeper context on related sizing topics, the antique tire sizes decoded piece does for vintage automotive what this article does for motorcycles, and the vintage tire speed ratings article covers the speed rating letter codes in more depth. If you're considering a bias-look radial as your replacement, the bias-look radial tires guide explains the construction trade-offs in detail. And the most popular vintage tire sizes overview is worth a read for context on which sizes have stayed in production and which haven't.

Key Takeaways

• Four sizing systems coexist on vintage motorcycles: standard inch (4.00-18), low-profile inch (4.25/85-18), alphanumeric (MT90-16), and metric (110/90-18). The standard inch system dominated pre-1970s, alpha appeared in the late '60s, and metric became the global default by the mid-1980s.
• Standard inch sizes assume ~100% aspect ratio. A 3.50-19 has roughly 3.5 inches of sidewall on a 19-inch rim. Low-profile inch defaults to 82% aspect ratio unless explicitly marked otherwise.
• Alpha widths follow a coded scale: MH=80mm, MJ=90mm, MM=100mm, MN/MP=110mm, MR=120mm, MT=130mm, MU=140mm, MV=150mm. Anything wider than 150mm uses metric only.
• The math doesn't always work. A 4.00-18 should convert to 100mm wide, but actually replaces with a 110/90-18 because vintage tire dimensions were nominal, not precise. Trust manufacturer cross-reference charts over your own arithmetic.
• Maintain mounted overall diameter within ~3% of original to keep speedometer accuracy and gearing feel intact.
• Verify rim width compatibility before buying a replacement. Vintage spoked rims are narrow (typically 1.60-2.15 inches), and modern wider tires can flatten incorrectly on them.
• TT means tube-type, TL means tubeless. Most vintage spoked rims require tubes regardless of the tire's TL rating.
• Construction codes: R is radial, B is bias-belted, no letter or a dash is standard bias-ply. Bias-look radials are radials with vintage-style tread patterns.

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