How To Find Out Offset Of Rims – A Simple Guide For Every Diyer

ByJim Boslice Metalworking & Fabrication

To find your wheel offset, look for the “ET” stamp on the back of the rim’s spokes or hub mounting surface. If no stamp exists, measure the total wheel width and the distance from the mounting surface to the center line of the rim.

Most gearheads and DIY enthusiasts know that upgrading wheels is the fastest way to change the look of a vehicle. However, picking the right set isn’t just about the diameter or bolt pattern. If you get the spacing wrong, you’re looking at rubbing fenders, damaged suspension components, or even dangerous handling issues.

I’ve spent plenty of time in the shop swapping out wheels, and I know how frustrating it is to have a new set of rims that just won’t clear your brake calipers. That is exactly why understanding wheel geometry is a skill you need in your workshop arsenal.

In this guide, I’m going to show you how to find out offset of rims so you can buy your next set with total confidence. We will walk through the visual cues, the measurement techniques, and why these numbers matter for your safety and vehicle performance.

Why Wheel Offset Matters for Your Vehicle

Before we grab the measuring tape, let’s talk about what “offset” actually means. Simply put, it is the distance between the mounting surface of the wheel—the part that touches your brake hub—and the center line of the wheel rim.

If the mounting surface is perfectly in the middle, you have zero offset. If the mounting surface is pushed toward the outside of the car, you have a positive offset. If it’s pushed toward the inner barrel, that is a negative offset.

Getting this wrong can ruin your day. Too much positive offset can cause your tires to rub against your struts or suspension arms. Too much negative offset pushes the wheels out, which might cause them to scrape against your wheel arches or fenders.

How to find out offset of rims by checking the stamps

The easiest way to find this information is to look for the manufacturer’s markings. Most modern aftermarket wheels and factory rims are stamped with their specifications directly on the back of the metal.

Where to look for markings

Take the wheel off the vehicle and lay it face down on a soft surface, like a rubber mat or a piece of cardboard. Look at the back of the spokes or around the hub area near the lug holes.

You are hunting for a code that starts with the letters ET, followed by a number. For example, you might see “ET35” or “ET45.” That number is your offset measured in millimeters.

If you don’t see an “ET” label, look for other markings like “Offset” or “Off.” Sometimes it is stamped as part of a larger string of numbers, such as “17x8J 35,” where 35 is the offset value.

Manual measurement: A step-by-step approach

Sometimes, wheels are older, aftermarket, or the markings have been painted over or worn away. If you can’t find the stamp, you will need to calculate it yourself. Don’t worry; it’s straightforward geometry.

Tools you will need

  • A flat, level surface (a metal workbench works great).
  • A straight edge (like a long ruler or a spirit level).
  • A tape measure or precision calipers.

The measurement process

  1. Place the wheel face down on your workbench.
  2. Measure the total width of the rim from the outer lip to the inner lip. Divide this number by two to find the center line.
  3. Lay your straight edge across the back of the rim (the side that faces the car).
  4. Measure the distance from that straight edge down to the mounting surface (the flat part where the lug holes are).
  5. Subtract the center line measurement from the distance you just measured. The resulting number is your offset.

If the mounting surface is further out than the center line, you have a positive offset. If it is closer to the inner lip, it is a negative offset.

Understanding the difference between offset and backspacing

When you are researching new wheels, you might run into the term “backspacing.” It is easy to confuse these two, but they are not the same thing.

Backspacing measures the distance from the mounting surface to the back edge of the rim, not the center. While offset is the industry standard for most modern cars, backspacing is often used by 4×4 enthusiasts and off-roaders.

If you are working on a truck or a specialized build, knowing both can be helpful. You can easily convert between the two if you know your rim width, but sticking to the offset measurement is usually safer for standard passenger vehicles.

Safety considerations for your garage project

Safety in the shop is non-negotiable. When you are testing wheels for fitment, never force a wheel onto the hub. If the offset is wrong, the wheel might sit slightly crooked or put uneven pressure on your wheel studs.

Pro-tips for safe fitment

Always perform a “dry fit.” Mount the wheel without the tire if possible, or use an old tire to check for clearance. Spin the wheel by hand to ensure it does not hit the brake calipers or suspension components.

If you are using wheel spacers to adjust your offset, make sure you have enough thread engagement on your lug nuts. A good rule of thumb is that the lug nut should thread on at least as many turns as the diameter of the stud.

If you are ever in doubt about whether a wheel fits correctly, don’t guess. Consult a professional mechanic or a reputable tire shop. A wheel that isn’t seated properly can lead to catastrophic failure while driving.

Frequently Asked Questions About Wheel Offset

Does offset affect my vehicle’s handling?

Yes, changing your offset significantly changes your “scrub radius.” This can make your steering feel heavier or twitchier. It also puts more stress on your wheel bearings, which could lead to premature wear.

Can I use different offsets on the front and rear?

Some sports cars use a “staggered” setup, but for most DIY daily drivers, it is best to keep the offset consistent across all four corners. This allows you to rotate your tires properly, which extends their lifespan.

What if my offset is slightly off?

If your new wheels have an offset that is within 3-5mm of your factory specs, you are usually fine. Anything beyond that requires careful measurement to ensure you won’t have clearance issues during hard cornering or bumps.

Is it possible to change the offset of a rim?

Technically, you can have a machine shop mill the mounting surface to increase positive offset, but this is dangerous and expensive. It is almost always better to buy the correct wheels from the start rather than trying to modify the metal.

Final Thoughts on Mastering Your Wheels

Learning how to find out offset of rims is one of those foundational skills that separates the casual garage tinkerer from the serious DIYer. It saves you money, keeps your vehicle safe, and prevents those “oops” moments that happen when you buy parts without doing the math.

Take your time with your measurements, double-check your clearance, and never be afraid to ask for help if a fitment looks too tight. The right set of wheels can completely transform your project, and now you have the knowledge to pick them with confidence. Keep wrenching, stay safe, and enjoy the process of perfecting your ride!

Jim Boslice
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