Wheel Offset and Backspacing Explained

DIY mechanics should take extra care to make sure they’re not either buying the wrong parts or inadvertently damaging their vehicles while installing new rims and tires. It’s important to install new custom tires correctly so that they won’t rub against your fenders. In the long run, a bad wheel installation can lead to long-term damage to the frame and suspension, among many other costly problems. The backspacing and wheel offset, explained here, are measurements car mechanics use to find and install wheels in the wheel well. We’ll be exploring what these measurements are, how they’re different, and how to use them.

What Does “Wheel Offset” Mean?

Offset measures the distance (in millimeters) from the mounting surface of the wheel to the center of the wheel. Basically, imagine an invisible line going through the exact middle of your wheel while looking at it from the side (long-ways rather than at the circle); that’s your wheel center. Then, the mounting surface (aka the back-pad or hub) is where your wheel meets your car. The distance between that imaginary line and that real surface is the offset. Why do we care about an invisible line? Well, this offset measurement affects whether the wheel will properly fit in your car or bump into the fender and suspension, as well as the aesthetics of your new rims.

• Positive Offset – Most wheels have a positive offset. That means that there is a positive distance between the wheel’s center and the mounting surface; the mounting surface will be closer to the outside of the car. Aesthetically, the rim will often appear to be smooth, without a large lip in the rim. Also, the tire as a whole will seem to fit tightly within the wheel well.
• Negative Offset – Wheels with a negative offset have a mounting surface that’s further inside than the wheel’s centerline, closer to the interior of the car. If you’re looking for a custom look, you might want to find wheels with a negative offset. What does offset mean on rims? Often the rims will have a lip and depth to them, having an appealing concave look. Also, the tire as a whole may seem to protrude out of the wheel well.

Backspacing vs. Offset

Basically, these two terms refer to the same thing measured two different ways. When the wheel is bolted to the hub, how far in or out should the rims and tires sit?

• Backspacing: Backspacing measures from the back of the wheel to the mounting surface. It’s an older system of measurement. It’s measured in inches. If your backspacing isn’t correct you might run into clearance issues – as the car turns, the wheel might hit the fender.
• Offset: Offset measures from that invisible middle line at the center of your wheel and the mounting surface. It’s measured in millimeters and considered to be slightly more exact.

How to Measure Wheel Offset

Often, you won’t have to!

This measure can be found on the back and inside of most wheels next to the letters “ET.”

But, in cases where the measurement is missing, here’s a basic rundown of how to find this measure:

1. Set down your wheel on the ground with the hub toward the floor and measure from the ground to the top of the wheel. You can make sure this is nice and accurate by using a second ruler across the top of the wheel or a piece of cardboard. (Note that this measure is different from the true wheel width you might see listed in the specs; that measure goes from bead to bead.)
2. Calculate your centerline. The measurement you just made divided in half is your center line. So, for example, if your measurement was 8 inches, the centerline would be 4 inches.
3. Find the backspacing. Measure from the mounting surface to the top of the wheel. Set the bottom of your ruler onto the hub and measure; again, you can use a second ruler or cardboard to get an accurate reading at the top.
4. Calculate the offset. Take that first measure, the wheel width, and divided by half to find the centerline. Then, subtract that number from the backspacing. For example, if the width I’d measured is 8 inches, my wheel centerline should be at 4 inches. Then, if my backspacing is 6 inches, 6 inches minus 4 inches is 2 inches. The offset is 2 inches, or about 50 millimeters.
5. If the number is higher than half of the width, this is a positive offset. If it’s lower than half the width, it’s a negative offset. For example, if your backspacing is 6 inches and the centerline was at 4 inches, that’s a positive offset. But if your backspacing was 3 inches and your centerline was 4 inches, that’s a negative offset.
6. If you were measuring in standard, don’t’ forget to convert it to metric. Offset is measured in millimeters. (Don’t be like NASA that one time!)

Backspacing vs. Wheel Offset Chart

If you’re trying to convert backspacing to offset or visa versa without measuring, and you know your stats already, use this chart to help you find the right wheel offset. (Remember that this is the true wheel width that measures from bead to bead, and different than if you measured it from the floor.)

If you’re doubting yourself while trying to figure out which tires to buy, give us a call! We can also help check your work with wheel and tire balancing services.