The hub cap is the small, round cover in the center of an alloy wheel. It closes off the hub bore—the hole through which the vehicle’s wheel hub would be visible—and typically measures between 50 and 75 mm in outer diameter, depending on the rim. The wheel cover, on the other hand, is a large, full cover that dresses up an entire steel wheel and is designed to look like an alloy rim. Both parts are on the wheel, both are made of plastic, both are constantly confused—but they have nothing to do with each other.
This guide shows you how to determine the correct size (the right measurement, not just any diameter), what types of attachments exist, why hub caps are more than just decoration, and what you can do if one gets lost on the road. We also clear up the most common confusion: hub caps vs. spigot rings.
Note: zrshop specializes in rims, wheels, and wheel accessories—we do not carry hub caps or wheel covers. Therefore, this article is purely a guide with no sales intent. Spigot rings, which are often confused with hub caps, are part of our product range—more on that below.
1. Hub Cap or Wheel Cover? The Difference in One Sentence
In short: The hub cap only covers the center, the wheel cover dresses up the entire wheel.
| Feature | Hub Cap (Center Cap) | Wheel Cover (Full Wheel Trim) |
|---|---|---|
| Sits on | alloy wheel (sometimes also steel wheel) | almost exclusively steel wheel |
| Covers | only the hub bore in the center of the rim | the entire wheel disc up to the rim flange |
| Typical Size | approx. 50–75 mm diameter | according to wheel inch size, usually 13–17 inches |
| Attachment | clip ring, screw-on, or bayonet | circumferential spring steel ring in the wheel disc |
| Covers wheel bolts? | no (only with very large caps) | yes, usually completely |
| Main Purpose | keep hub and wheel fastening clean | aesthetics and protection of the wheel disc |
There is a third variant that completely adds to the confusion: the hub cap for steel wheels. Some steel wheels are driven without a full wheel cover and instead only have a small cap in the center—functionally a hub cap, mounted on a steel wheel. So if you are looking for "hub caps for rims", first clarify whether you mean the small center cap or the large wheel trim. Everything else depends on that.
2. Measuring Hub Cap Size: What Measurement Really Matters
The most common mistake when buying replacements: some random diameter is measured – and then it doesn't match the measurement the rim manufacturer specifies. A hub cap has at least three different diameters, and they differ by several millimeters.
| Measurement | Where to measure | What it determines |
|---|---|---|
| Outer diameter | across the outermost edge of the visible cap surface (front) | whether the cap sits flush in the center of the rim for aesthetics |
| Clip diameter | across the outer edges of the clip ring on the back | whether the cap holds at all – the actual fitting dimension |
| Rim seat diameter | inner diameter of the cap receptacle, measured from the front in the center of the rim | reference if no cap is present |
| Overall height / Installation depth | from the back of the clip ring to the visible surface | whether the cap protrudes, sits flush, or sinks too deep |
| Hub bore (CB) | centering seat on the back of the rim | nothing at all for the cap – this is the spigot ring size |
Here's how to proceed if you still have a cap: Take an intact cap off the other wheel – this is always the most reliable template. Use a caliper to first measure the outer diameter of the visible surface, then the clip diameter across the spring-loaded tabs. Note both values to one tenth of a millimeter. Then look at the back: many caps have an embossed part or item number and the manufacturer's logo there. This number is more valuable than any measurement because it uniquely identifies the cap.
Here's how to proceed if no cap is present: Measure the inner diameter of the round opening in the center of the rim from the front. It is important to hit the correct step: The cap receptacle is its own, usually slightly conical bore on the visible side of the rim and, as a rule, larger than the hub bore on the rear of the wheel. If you accidentally measure the hub bore, you will end up with a value that is much too small. Therefore, measure twice – once at the front of the cap step, once at the back of the centering seat – and use the front value.
Why Manufacturer Specifications Differ
The fact that two sources quote different millimeter figures for the same cap has tangible reasons:
- Different reference edges. One manufacturer specifies the outer diameter of the visible surface, the next the clip dimension. Depending on the design, there are 2 to 5 mm between the two.
- Rounded nominal dimensions. A cap with 64.7 mm is often listed as "65 mm". This is not sloppiness, but common nominal size practice – you just need to know it.
- Brand-specific cap seats. Each rim brand designs its own hub receptacle: number and shape of the clip tabs, depth of the groove, angle of the contact surface. Two caps with identical outer diameters can therefore have completely different backs.
- Manufacturing tolerance and temperature. Plastic works. A cap that fits snugly at 20 °C will be noticeably harder to clip in when frozen.
As a rule of thumb: a deviation of about half a millimeter is still compensated by the clip ring. One millimeter too small, and the cap will fall out at the first curb. One millimeter too large, and it can only be inserted with force - in which case the clip tabs typically break.
Common Size Ranges
The following overview categorizes the dimensions that frequently appear in practice. These are approximate values, not assignments to specific vehicles – you should always measure yourself.
| Range | Outer Diameter | Typical Clip Dimension | Where it's found |
|---|---|---|---|
| small | approx. 50–56 mm | approx. 45–52 mm | small car wheels, delicate accessory rims |
| medium | approx. 57–65 mm | approx. 54–61 mm | by far the most common range for passenger car alloy wheels |
| large | approx. 66–70 mm | approx. 62–66 mm | larger passenger car and SUV wheels |
| very large | approx. 71–78 mm | approx. 67–74 mm | luxury class, transporter, and many off-road wheels |
Example Mercedes: why “75 mm” is only the beginning
If you're looking for a Mercedes hub cap, you'll almost always first come across the value 75 mm. This outer diameter is indeed widely used across many generations of the brand's alloy wheels and has become a kind of unofficial standard. However, you shouldn't rely on it: significantly smaller caps in the 66–67 mm range are also circulating on the brand's vehicles, transporter and commercial vehicle wheels differ, and accessory rims from other manufacturers naturally come with their own cap seat – even if they are mounted on a Mercedes.
What's crucial is not the vehicle brand, but the rim brand. If an accessory rim is on the car, the cap depends on its design, not on the star on the hood. Conversely: Even with an original rim, the cap differs depending on the model series and year of manufacture. So measure it yourself instead of ordering by vehicle brand. The same applies analogously to all other vehicle brands.
View spigot rings in all common sizes in the zrshop →
3. Mounting Types: Clip, Screw, Bayonet
If the size is correct, but the cap still doesn't fit, it's almost always due to the mounting type. Three systems are common.
| System | How to identify it | Installation | Limitations |
|---|---|---|---|
| Clip system | spring-loaded plastic tabs or a circumferential clip ring on the back | press in evenly with the palm of your hand until it clicks audibly into place | tabs fatigue with each removal; in frost, the plastic becomes brittle and breaks more easily |
| Screw cap | thread on the cap edge or back with screw holes | screw in by hand until it stops, or screw from the back | do not overtighten the thread – plastic threads tear easily; clean the seat before screwing in |
| Bayonet | side tabs or lugs that run into ramps | insert, press lightly, and twist until it stops | only works with the brand's specific counterpart; twisted tabs break off |
A fourth variant is found mainly on off-road and pickup wheels with large hub bores: there, the cap is occasionally inserted from the rear of the wheel and secured with a snap ring or a retaining plate. The advantage is a very firm fit, the disadvantage: the wheel must be removed to change it. If you drive such wheels, it makes sense to check the cap fit during the seasonal wheel change.
Note: Never use a hammer. A cap that cannot be inserted with even hand pressure is either too large or misaligned. Blows almost always hit the clip tabs – and these are the part that holds the cap. For removal, a plastic lever or a screwdriver wrapped in a cloth, applied to the groove on the edge of the cap, is suitable.
4. Hub Caps Are Not Just for Looks
A hub cap is not a safety-relevant component – the car drives just as well without it. Nevertheless, it has a tangible technical function: It keeps water, road salt, brake dust, and grit out of the hub area. This is exactly where the parts are located that you want to loosen again at the next wheel change.
- Wheel bolts and lug nuts. With large caps, the bolt heads are located under the cap. If they stay dry, the Allen key or hexagonal head will not get clogged with rust and salt crust – the socket can be attached cleanly, and the head will not round off.
- Centering seat and wheel hub. The bare contact surface between the hub and rim corrodes under the influence of salt. A wheel that has become stuck can only be removed with difficulty by the roadside – an annoyance that a 3-euro part could often have prevented.
- Spigot rings. Where a spigot ring is installed, it sits directly behind the cap. If the area remains clean, the ring can be easily removed and reused at the next change. The comparison Spigot rings: Material differences shows which materials are how robust.
- Cleaning. A closed hub area can simply be cleaned faster. Practical tips on this can be found in the guide Cleaning and caring for rims.
Conversely, if you drive a wheel for a longer period without a cap, you should consciously inspect the hub area during a wheel change, clean it, and remove any rust from the contact surfaces. Driving without a cap is not prohibited – there are no regulations that require hub caps. It is only important that no sharp-edged fragments remain: According to § 30c StVZO (German Road Traffic Licensing Regulations), vehicle parts must not have avoidable sharp edges or protruding external parts that could injure other road users. Therefore, a half-torn cap should be removed completely, not left partially attached.
5. Wheel Covers for Steel Wheels: Fit and Security
Wheel covers are the domain of the steel wheel. They cover the wheel disc, protect it from dirt, and make the wheel appear more valuable. The fit is primarily determined by the wheel's inch size: A 16-inch cover belongs on a 16-inch steel wheel, period. However, the inch size alone is not enough.
- Wheel disc contour. Steel wheels differ in disc depth and spoke pattern. A cover that sits flush on one 16-inch wheel may stand proud on another.
- Valve cutout. The cover needs a cutout for the valve. With the rigid metal valves of modern tire pressure monitoring systems, this is not a detail, but a prerequisite for the cover to lie flat at all.
- Clearance for the wheel bolt. With clip-on trim caps or longer wheel bolts, it can get tight inside.
Why Wheel Covers Come Loose
A full wheel cover is almost always held by a circumferential spring steel ring on the back, which clamps into the wheel well. This ring is the weak point. Typical causes for lost caps are a fatigued or bent tensioning ring after several seasons, contact with the curb when parking, the high-pressure jet in the car wash, continuous vibration on poor roads, and – most commonly – simply the wrong size or contour.
When mounting, a system helps: align the cap so that the valve cutout is exactly over the valve, then press evenly all around with the palm of your hand – first opposite points, then the spaces in between. If you only press in one spot, the cap will tilt and stress the ring unevenly. As an additional securing measure, a thin cable tie through a spoke slot is common in practice. Make sure that the cut end is flush and no edge protrudes outwards – see again § 30c StVZO.
A point that often surprises: during the main inspection, the wheel attachment must be assessable. An officially recognized inspection organization can therefore demand that full wheel covers be removed. Covers that can be removed non-destructively are clearly an advantage here.
If you're already thinking about steel wheels for the cold season: the winter complete wheels guide explains the arguments for steel and for alloy. Fully assembled sets can be found under Rims, and the matching wheels themselves under Steel Rims.
6. Hub Cap or Spigot Ring? The Most Common Confusion
Both are round, both are usually made of plastic, both are specified in millimeters, and both are located in the hub area. Nevertheless, they are completely different components with completely different tasks:
| Hub Cap | Spigot Ring | |
|---|---|---|
| Task | closes off the hub bore to the outside | centers the rim on the wheel hub |
| Position | on the visible side, at the front in the center of the rim | invisible between the wheel hub and the rim |
| Visibly mounted? | yes | no |
| Measurement | one diameter (e.g., 65 mm) | two diameters (e.g., 72.6 → 66.6 mm) |
| Technical Relevance | protection, appearance | concentricity, vibration-free, relief of wheel bolts |
| Can be omitted? | yes, without technical consequences | only if the hub bore fits exactly |
The distinguishing feature in practice is the size specification: A hub centric ring is always described with two values – outer diameter (the hub bore of the rim) and inner diameter (the centering seat of the vehicle). If only one number is given, it refers to a cap. The guide Perfect Centering with Hub Centric Rings explains how centering works and why it is important for vibration-free running; when you can do without it and when not is explained in Driving Rims Without Hub Centric Rings. The Hub Centric Ring Guide provides a compact overview of selection and use.
Note: A missing hub cap never replaces a missing hub centric ring. If you're driving an accessory rim with a larger hub bore, you need the ring – regardless of whether there's a cap on the front or not.
7. Lost Hub Cap: How to Find a Replacement
A cap is usually lost unnoticed: curb, pothole, car wash. When buying a replacement, it's not the vehicle brand that matters, but the rim brand and rim model. The reason is the brand-specific cap seat from section 2 – the same outer diameter does not mean the same seat. Proceed in this order:
- Use the second cap as a template. An existing cap from the same wheel set provides measurements, design, and usually a part number on the back.
- Determine the rim brand. The logo is often embossed into the rim star or found on the back of the wheel.
- Read the rim data from the back. This usually includes size, offset, and identification numbers. The guide to the KBA number on rims explains how to interpret this information.
- Measure both diameters. Outer and clip dimension, plus installation height – this allows you to check any offer specification, regardless of what dimension it refers to.
- Don't forget the cap height. There are flat and curved designs. A cap that is too high will protrude and be lost at the first car wash; one that is too flat will disappear into the bore.
A common special case: For a used wheel set, the caps sometimes do not come from the mounted rim at all, but are spare parts from another source. In this case, the part number on the back of the cap is misleading – rely on the measured dimensions and the rim brand, not the embossing.
And another practical tip for off-road and pickup wheels: The hub bores and thus also the caps are often significantly larger than on car rims, and some models deliberately do not have a cap because the open hub is part of the design. If you are planning such wheels, you will find suitable models under Off-road rims – and should regularly clean and inspect the hub area during off-road use anyway.
FAQ: Hub Caps and Wheel Covers
How do I correctly measure the size of a hub cap?
Measure two values with a caliper: the outer diameter over the outermost edge of the visible surface and the clip diameter over the lugs on the back. If no cap is present, measure the inner diameter of the cap receptacle from the front in the center of the rim – not the hub bore at the back of the wheel, which is smaller. Also note the installation height so that the cap neither protrudes nor sinks.
What is the difference between a hub cap and a hub centric ring?
The hub cap closes the hub bore on the visible side of the rim and is purely a protective and decorative part. The hub centric ring sits invisibly between the wheel hub and the rim and compensates for an oversized hub bore to the centering seat of the vehicle – it is responsible for the concentricity. Distinguishing feature: Hub centric rings are always specified with two dimensions (outer → inner), hub caps with only one.
May I drive without a hub cap or wheel cover?
Yes. Neither hub caps nor wheel covers are mandatory components; driving without them is permitted. Just make sure that no broken remnants with sharp edges remain on the wheel – according to § 30c StVZO, vehicle parts must not have avoidable sharp-edged or protruding external parts. In the long term, the open hub area will get dirtier and corrode more, which makes the next wheel change more difficult.
What hub cap size fits Mercedes?
An outer diameter of around 75 mm is very common for alloy wheels of the brand, in addition to smaller caps around 66–67 mm and deviating dimensions for vans. However, it is not the vehicle brand that is decisive, but the rim brand: On an accessory rim, the cap of this rim brand sits, regardless of the vehicle. Therefore, always measure yourself instead of ordering solely by vehicle brand.
Legal Status and Sources
Related
Note: Legal status and review date August 18, 2026. Information without guarantee; the currently valid version of the mentioned regulations is always authoritative. The mentioned size ranges are empirical values from practice and do not replace your own measurements. Vehicle and rim brands are mentioned for descriptive purposes only.
- Road Traffic Licensing Regulations (StVZO) – § 30 Condition of Vehicles, § 30c Protruding Outer Edges, § 36 Wheels and Tires
- Fundamentals of Wheel Mounting – Hub Centering, Centering Seat, Wheel Bolt Connection
- Own Measurement Practice on Car, SUV, and Off-road Wheels (Outer, Clip, and Seat Diameter, Installation Height)