Calculating wheel offset is the most challenging part of any wheel conversion. Many enthusiasts have a basic understanding of wheel offset but get lost in the numbers. The root of their confusion lies in the massive amount of misinformation on the web. In order to properly configure a set of wheel adaptors, wheel offset must be completely understood. Let's start at the beginning with a basic overview of wheel offset. To help visualize we cut a 16" OEM wheel in half and painted the cross section blue.
Wheel offset is a measurement of the distance between the mounting surface and the true center of a wheel's width. In the picture above, the wheel's mounting surface, also known as mounting pad, is shaded lime green. A dark red line representing the centerline of the wheel has been painted dark red on the actual wheel. The centerline of the wheel represents the true center of the width of the wheel. The distance between the centerline and the mounting pad has been shaded a light red. This shaded area is the wheel offset.
Measurement of wheel offset can be positive or negative and is described in millimeters. An offset of 0 describes a wheel mounting pad positioned in the center of the wheel. A negative offset indicates the mounting pad is is positioned on the hub side of the centerline, closer to the vehicle. A positive offset indicates the mounting pad is positioned towards the face of the wheel. In the picture above, a + and - have been drawn in yellow on either side of the wheel centerline. The wheel pictured has a positive offset, proven by the position of the mounting pad.
Nearly every wheel has the offset stamped on it somewhere. Typically, the number is stamped on the back of the spokes. It can be cast into the face, stamped into the barrel, or even printed on the outside of the barrel. If a wheel doesn't have the offset clearly stamped, it can be easily measured. Using our wheel cross section we did a basic measurement of wheel offset to confirm the number stamped. First, we measured the distance between the inside of the lips and marked the center. Next, we sat a straight edge on the mounting pad and extended it out to touch the barrel. We marked the bottom of the straight edge under our centerline mark. A simple measurement between these two marks gave us our wheel offset. This wheel is marked ET50. We measured 50mm.
Wheel offset is commonly misunderstood and can often be misrepresented. Here's a few things to look out for when asking for accurate offsets.
- Wheel offset is commonly known as ET, an abbreviation for a German word meaning "press depth."
- Most wheels are stamped with their offset using ET, however some use the prefix IS. ET50 = IS5
- Watch out for offsets to be mislabeled + or - . Many people don't know the difference between a positive and a negative offset. They see ET50 and think -50 due to something they read online. If in doubt - always ask for pictures of the offset stamp and the back of the wheel.
The most confusing part of wheel offset is comparing wheels. This is where your next door neighbor has been confusing you for all these years. No matter what wheels you have, what wheels you want, or what wheels your buddy has, offset is relative to your car, and your car only. Just because a wheel has an interesting offset doesn't mean it will actually fit your car.
If your neighbor with a pickup truck is telling you to get some "low offset wheels" he's thinking ET-20. If your current wheels are ET20 and you follow his advice, you just tried to install wheels that stick out 40mm more than your current wheels... and it didn't work.
When someone says you need a "higher offset," what do they mean? Higher in number towards the wheel? Higher in number towards the hub? Higher compared to what? This is a rhetorical question. The answer is based on the specs of your current setup. Nothing else.
To determine if another wheel may fit, a series of measurements and calculations need to be done using existing data and data from the prospective wheel.
What Size Adaptors Will I Need To Fit These Wheels?
Now that you have a solid understanding of wheel offset we can dive into the math. Next we're going to walk you through a step-by-step method of determining if your dream wheel will even fit and what size adaptor to use. Before you grab your calculator, realize that this method requires ACTUAL numbers and ACTUAL measurements. Guessing will result in very poor fitment.
First we need to establish a base measurement. This measurement will be known as your clearance factor and will be the measurement to which all prospective wheels will be compared. To determine the clearance factor of your existing wheels, you will need the stated width and ET of your existing wheels. Calculate front and rear wheels separately.
(wheel width / 2) x 25.4 - ET +10 = clearance factor
This equation divides the width of your wheel in half to find the centerline. This measurement is almost always in inches, so multiplying by 25.4 converts the width to millimeters. Next, the offset is subtracted from the measurement. 10mm is added to represent the thickness of the front lip. Using a stock Mk5 GTI wheel and this equation we came up with a clearance factor of 44.25mm
17"x7.5" ET51: (7.5" / 2) = 3.75" x 25.4 = 95.25mm - 51mm = 44.25 + 10mm = 54.25mm
This is the actual distance between the hub and the outside of the lip.
Next, we need to get a feel for how the current wheels fit on the car. The only way to do this is to measure! What we're looking for is a measurement of the distance between the face of the wheel and the inside of the fender.
Head out to your car with a straight edge and a metric ruler. Park your car on flat pavement and straighten your front wheels. Lay your straight edge against the face of one wheel. It should be vertical, crossing through the center of the wheel until it touches the fender.
If your straight edge sits on the tire and doesn't touch the face of the wheel, measure the distance between the straight edge and the lip of the wheel. The lip of the wheel is our reference point.
Next, use a metric ruler to measure the distance between the inside of the straight edge and the inside of the fender. If your straight edge doesn't touch the wheel, add the distance measured earlier.
Pictured above, the straight edge did not sit flat on the face of our wheel. We measured a distance of 12mm to the very inside of the fender and added 4mm to reach the lip of the wheel. Combined, we now know that we have 16mm of space between the lip of the wheel and the inside of the fender.
Next, we add this measurement to our clearance factor to get a total clearance factor of 70.25mm
54.25mm + 16mm = 70.25mm
When measuring for clearance, take the time to measure front and back. It's important to know the exact measurement of both sets of wheels when choosing adaptor thickness. The measurements taken above were on the front wheels.
Now the fun part... If you've already picked out new wheels, run your width and ET through the equation to find your clearance factor.
(wheel width / 2) x 25.4 - ET +10 = clearance factor
We're looking to install front wheel that measures 18"x8" ET60. Using our equation, we came up with a clearance factor of51.6mm.
18"x8" ET60: (8" / 2) = 4" x 25.4 = 101.6mm - 60mm = 41.6mm + 10mm = 51.6mm
Next we add the adaptor. In this case the clearance numbers are pretty close, so adding an adaptor will push the wheel out further than stock. Of course we want to push the wheels out, but not too far. To add an adaptor to the measurement, simply add the thickness of the adaptor to the clearance factor of your new wheel.
new wheel clearance factor + adaptor thickness = total estimated clearance
51.6mm + 15mm = 66.6mm
Using a 15mm adaptor, we ended up with a total estimated clearance of 66.6mm. Compared to the stock wheel, our new wheel will stick out ~12mm more. This is 4mm under our total clearance to the fender. Success!
Six Reasons Why They Won't Fit
This calculator is only meant to get you started. There are several limiting factors that come into play when pushing the limits of wheel fitment. Just because the math looks right doesn't automatically mean they fit. Here's a few factors to consider:
- When measuring for total clearance, it's best to take these measurements at the same ride height you plan to use the new wheels. Are you looking at a set of coilovers? Lower it before you buy the wheels. Although this method should produce an accurate measurement at any ride height, it's highly recommended to measure at your final ride height.
- On the topic of ride height - how low are you planning to go? It's tough to lower a car 4" over a 10" wide wheel without sitting the fender on the tire. Be realistic. If you want to go wide, you can't go low without rubbing hard. If you want to lower it to the pavement, you can't go wide. Pushing the limits of wide and low is why we do this, but there are limitations.
- Suspension geometry pays a major role in wheel fitment. Wheels don't just gracefully float up and down in the body when you hit a bump. Wheel camber, toe, and the overall characteristics of your suspension travel will dictate when and where a wide wheel will hit.
- Fender geometry differs on every car, front and back. The shape of the sheet metal defines how wide and how low you can go. The clearance measurements we suggest start at the inside of the fender. To gain more clearance, fenders can be pinched, rolled, and stretched. Almost all of this requires paint work, so measure carefully before making a financial investment in fender work.
- Tire size is critical to fitment. The clearance measurements we suggest are based on the lip of the wheel. We're assuming you're going to run a wide wheel and a slightly undersized tire to avoid hitting the fenders. In many cases, tire size determines fitment. The more narrow the tire, the more outside clearance you'll achieve.
- Last, but not least is wheel width. We have no way to tell you how wide you can go before hitting suspension components with the inside of the wheel. To avoid interference the wheel width and offset must be matched to position the back of the wheel away from the suspension. Also, a general warning about wheel width. If it won't fit, it's not going to fit. You can't run an 11" wide front wheel where a 7.5" wheel used to roll. It's just too big.
Realistically, we cannot help you with any of the six fitment aspects listed above. Throughout our website we have given you an excess of technical information to help with your wheel conversion. In the end, the aspects of your unique vehicle and prospective wheels determine the success of your fitment. We can guide you, but we can't make the final decisions. We suggest searching the internet for people who have installed the same wheels. Google has a wealth of information. Search the forums and don't be afraid to ask for help!