If you ask most drivers, "What's the difference between 16" and 16.5" sized tires and wheels?" They will answer, "half an inch." And while their answer is partially correct, it doesn’t address any of the other major differences that exist whenever tire size and wheel diameters change.
Every tire, and the wheel upon which it is mounted, must fit together better than a hand in a glove. Tire bead diameters and profiles must exactly match their specified wheel's bead seat dimensions and contours.
The Tire and Rim Association, Inc. (T&RA) represents tire and wheel manufactures in the United States, while similar regional associations represent manufacturers throughout the rest of the world. Collectively, all these regional organizations have established global tire and wheel engineering standards.
The following tech article describes various tire sizes along with the wheel diameters and bead seat contours that have been used over the past decades, as well as the vehicle types to which they were applied.
The most popular cars and light trucks tire and wheel sizes are commonly referred to as Inch-Rim sizes because they feature rim diameters in whole inches, such as 16", 17", 18", etc. However because they are common, most drivers don't realize these sizes have less known additional requirements. Inch-Rim sizes must also feature an ISO* J contour with a 5°-bead seat taper to ensure an air-tight seal, bead seat safety humps to help retain beads in case of inflation pressure loss, tall wheel flanges to laterally support tire sidewalls and a drop center to aid in ease of mounting.
Typical Car Inch-Rim Size Cross Section
Typical Car Inch-Rim Bead Specifications – P205/55R16 Size
Motorcycles evolved along a similar timeline as the automobile and share the basic Inch-Rim size designation and 5°-bead taper. However, motorcycle dynamics involve leaning when changing direction or going around a corner, making their dynamics considerable different than automobiles which remain relatively level. Motorcycle wheels have additional specifications that require a different contour with considerably narrower bead seats and lower bead flange height to accommodate a motorcycle tire's very round profile.
Typical Motorcycle Inch-Rim Size Cross Section
Typical Motorcycle Inch-Rim Bead Specifications – 200/55R16MC
Warning: Tires designed for four-wheel, dual-track automobiles are not interchangeable with tires nor wheels engineered for two-wheel, single-track motorcycles. In addition to a motorcycle tire's rounded profile vs. an automobile tire's relatively flat profile affecting tire footprint distribution and handling, automobile tires mounted on wheels meeting motorcycle design standards cannot correctly seat due to the motorcycle wheel's 20% narrower bead seat, nor are they properly supported laterally by the motorcycle wheel's 20% shorter bead flange. Due to the risks the rider is exposed to, Tire Rack will not sell, install, nor service tires for Darkside motorcycle riders.
Heavy-duty and commercial trucks, as well as many recreational vehicles and over-the-highway busses, feature sizes which use high inflation pressures to carry heavy loads while accommodating removal, repair and/or replacement by mobile tire services along the side of the road. To differentiate these heavy-duty tires from those used for passenger vehicle applications, these sizes have come in size diameters ending in a half inch, such as 16.5", 19.5", 22.5", etc.
Because these vehicle applications are engineered to use high inflation pressures to carry heavy loads, they use wide, 15°-bead seat tapers to ensure an air-tight seal and to accommodate tires with either single or dual inboard/outboard beads, as well as a wheel drop center to aid in ease of mounting.
Typical 16.5" Wide Base Wheel Cross Section
Typical Heavy-Duty Truck, Bus and Wide-Base ½"-Rim – 9.50R16.5LT
Warning: Once used as Original Equipment on heavy-duty-model consumer light truck vehicles in the 1960's and 1970's, continued use of 16.5" size applications as Original Equipment on new consumer vehicles is now prohibited. The 16.5" size tires should never be considered equivalent or interchangeable with 16" Inch-Rim sizes, nor should any ½"-rim diameter sizes be considered equivalent or interchangeable with standard inch-rim sizes.
TR Metric sizes features a proprietary Michelin engineered metric wheel-tire combination that is not interchangeable with conventional Inch-rim sized wheels and tires. Michelin's TRX tires were used as Original Equipment during the 1970's on selected car models produced in America and Europe. The TR name stands for "Tension Repartie" (distributed tension) with the tire casing being more elliptical in cross section. This helped allow TR Metric sized tires to combine lower aspect ratios to retain traction, handling and ride quality along with slightly higher inflation pressures to reduce tire rolling resistance.
TR Metric sizes featured unique contours with lower angled flanges. To help prevent Inch-Rim sized tires from being mounted on TR Metric wheels, or TR Metric tires from being mounted on Inch-Rim sized wheels, TR Metric sizes featured rim diameters measured and expresses in millimeters (i.e., 365mm, 390mm or 415mm) with bead diameters about ⅓-inch larger than the nearest whole inch.
Typical TR Metric Wheel Cross Section
Typical TR Metric Sizes Bead Specification – 220/55R390
Metric "A" (Asymmetric) sizes features a proprietary Michelin engineered metric wheel-tire combination that is not interchangeable with conventional Inch-rim sized wheels and tires. Michelin PAX tires were used as Original Equipment during the 2000's on selected car and van models produced in America and Europe.
Metric "A" (Asymmetric) sizes are used in conjunction with Michelin PAX Run-Flat tires which also feature an auxiliary internal support ring and unique rim bead seat contours that allow PAX tire beads to help retain grip of the Metric "A" wheel flanges, even after partial or complete pressure loss. To facilitate installation of the internal support ring during tire mounting, Metric "A" Sizes feature two different diameter bead seats. For example, a 460A designated rim features a 450mm outboard bead diameter and a 470mm inboard bead diameter.
Typical Metric "A" Wheel Cross Section
Typical Metric "A" Sizes Bead Specification – 220/55R390
Car and motorcycle tire sizes (which feature different bead seat widths and flange heights) were developed around the Inch Rim with its 5° taper. However, every other time the tire/wheel bead profile and contour was changed, the size's rim diameter nomenclature was also wisely changed. This was purposely done to help reduce the possibility of a consumer or service provider accidently attempting to mis-mount incorrect combinations.
If you compare the popular Inch-Rim dimensions and bead contours to the other tire size type's diameters and contours, the multiple differences in design specifications confirms the absence of interchangeably.
| Rim Diameter | Bead Set | ||||
|---|---|---|---|---|---|
| Outboard | Inboard | Width | Taper | Flange Height | |
| Car Inch-Rim | 16" | 16" | 19.8mm | 5° | 17.3mm |
| Motorcycle | 16" | 16" | 16.0mm | 5° | 14.0mm |
| Wide-Base | 16.5" | 16.5" | 25.0mm | 15° | 14.2mm |
| TR Metric | 390mm (15.35") | 390mm (15.35") | 20.3mm | 5° | 14.2mm |
| Metric "A" | 450mm (17.71") | 470mm (18.50") | 13.2mm | 3-sided | 6.2mm |
Care must always be taken to confirm the existing tire/wheel size in use when buying and mounting replacement tires or wheels. After all, the complete differences between a 16" and a 16.5" sizes are much greater than just the "half an inch" difference in their diameters.
*International Standards Organization (ISO)
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