To read rim size, locate the alphanumeric code stamped or embossed on the rim’s inner barrel or spoke. This code follows a standard format: diameter x width, offset, and bolt pattern. For example, “17×7.5 ET45 5×112” indicates a 17-inch diameter rim with 7.5-inch width, 45mm offset, and five bolt holes on a 112mm circle.
Understanding rim size markings
Rim size information appears in several locations on your wheel. The most reliable spot is the inner barrel of the rim, where manufacturers stamp permanent markings during production. These markings remain visible even after years of use and weather exposure.
The standard format includes several key measurements. The first number represents the rim diameter in inches, measured from one edge to the opposite edge. The second number indicates the rim width, also in inches, measuring the distance between the inner flanges where the tyre beads sit.
Decoding the rim size code
A typical rim marking might read “18×8.5 ET35 5×120”. Breaking this down: 18 represents the diameter in inches, 8.5 shows the width in inches, ET35 indicates the offset in millimetres (how far the mounting surface sits from the wheel’s centreline), and 5×120 describes the bolt pattern (five bolts on a 120mm diameter circle).
Some manufacturers use different abbreviations. Instead of “ET”, you might see “OFFSET” or just a number with a plus or minus sign. German manufacturers often use “ET” (short for Einpresstiefe), whilst American companies typically spell out “OFFSET”.
Common rim marking locations
Beyond the inner barrel, rim size information appears in several places. The spoke faces sometimes display partial information, particularly on aftermarket wheels where manufacturers want to showcase their specifications. However, these markings can wear off over time.
Original equipment wheels often have additional markings on the rear face. These include part numbers, manufacturing dates, and quality certifications alongside size specifications. Check behind the wheel when it’s removed for maintenance.
Professional tip: Our AutoPartsPro mechanics recommend photographing rim markings before wheel removal. This creates a permanent record for future reference and helps when ordering replacement wheels or tyres.
Alternative methods for determining rim size
When rim markings are illegible or missing, several alternative approaches work effectively. Your vehicle’s owner’s manual contains original equipment wheel specifications. This information helps verify compatibility when considering aftermarket wheels.
Online databases using your vehicle identification number (VIN) provide accurate original specifications. Many manufacturers maintain searchable databases where you can input your registration number or VIN to access complete wheel and tyre specifications.
Physical measurement remains possible but requires wheel removal for accuracy. Measuring diameter involves placing a straight edge across the rim and measuring the distance. Width measurement requires checking the internal distance between bead seats.
Reading offset and backspacing
Offset markings deserve special attention as incorrect offset affects vehicle handling and brake clearance. Positive offset (indicated by a plus sign or just “ET” followed by a number) means the mounting surface sits towards the wheel’s face. Negative offset places the mounting surface towards the back.
Some wheels display backspacing instead of offset. Backspacing measures the distance from the mounting surface to the wheel’s rear edge. Converting between offset and backspacing requires knowing the total wheel width.
Bolt pattern identification
Bolt pattern markings follow a standard format showing the number of bolts and the diameter of the circle they form. Common patterns include 4×100, 5×112, 5×120, and 6×139.7. The first number indicates bolt quantity, whilst the second shows the pitch circle diameter in millimetres.
Some manufacturers use imperial measurements for bolt patterns, particularly on older American vehicles. These appear as fractions (like 5×4.75) rather than decimal millimetres.
Expert advice: According to our wheel specialists, always verify bolt pattern measurements rather than assuming compatibility. Even similar-looking patterns can differ by crucial millimetres, leading to dangerous fitment issues.
Special markings and certifications
Quality wheels display various certification marks alongside size information. These include load ratings, maximum pressure specifications, and safety certifications from organisations like TÜV or DOT. Understanding these markings helps ensure proper wheel selection for your vehicle’s requirements.
Date codes indicate manufacturing periods, useful for warranty claims or safety recalls. These typically appear as week/year combinations (like “2823” for the 28th week of 2023) near other rim markings.
Common rim size formats by manufacturer
Different manufacturers use varying formats for displaying rim information. European brands typically follow metric measurements with clear ET offset markings. Japanese manufacturers often combine metric and imperial measurements on the same wheel.
Aftermarket wheel companies frequently add decorative elements around size markings. These might include brand logos, model names, or design elements that can obscure critical size information. Look for the standardised alphanumeric codes rather than decorative text.
Digital tools and apps
Modern smartphone applications can assist with rim size identification. These apps often include databases of vehicle specifications and barcode scanners for manufacturer part numbers. However, they shouldn’t replace physical verification of rim markings.
Professional tyre shops use sophisticated measuring tools for precise rim sizing. These tools provide accuracy beyond manual measurement methods, particularly useful when dealing with damaged or worn rims where markings might be unclear.
Troubleshooting unclear markings
Corrosion, paint, or damage can obscure rim size markings. Gentle cleaning with appropriate solvents might reveal hidden information. Avoid aggressive cleaning that could damage wheel finishes or protective coatings.
Wire brushes or steel wool can remove stubborn corrosion from cast wheels but shouldn’t be used on painted or polished surfaces. For valuable wheels, professional restoration might be worth considering to preserve both function and appearance.
When markings remain illegible, professional wheel identification services exist. These specialists can identify wheels through design characteristics, manufacturing details, and comparison with extensive databases.
Rim size markings are typically stamped on the inner barrel of the wheel, behind the spokes. Some wheels also display information on spoke faces or the rear mounting surface.
ET stands for “Einpresstiefe” (German for offset depth). It measures in millimetres how far the mounting surface sits from the wheel’s centreline, affecting vehicle track width and handling.
Basic diameter can be estimated whilst mounted, but accurate width, offset, and bolt pattern measurements require wheel removal. Check your owner’s manual for original equipment specifications instead.
Quality wheels from reputable manufacturers always include size markings. If markings are missing, the wheel may be damaged, counterfeit, or from an unreliable source.
Rim size refers to wheel dimensions (diameter, width, offset), whilst tyre size includes sidewall height and width. Both must match for proper fitment and performance.
Understanding rim size markings empowers you to make informed decisions about wheel purchases, replacements, and upgrades. Whether you’re dealing with original equipment wheels or considering aftermarket options, accurate size identification ensures proper fitment and optimal vehicle performance. When in doubt, consulting with wheel professionals or referencing manufacturer specifications provides the reliability needed for safe driving.
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