In this blog post we are going to do our best to explain what a wheel offset is, also called ET-measurement, so let us unravel the mystery surrounding this often asked question. By the way, the abbrevation "ET" actually comes from the german word Einpresstiefe which means "Insertion depth" in english, now you know!
So, if we continue on, the wheel offset (or ET-measurement) is the distance from the center of the rim (if we would cut the width of the rim in two equal sizes) to the contact surface where the rim mounts to the wheel hub of the car, easy to understand huh?
Let's go over it again, even more thorough: The wheel offset (ET is another word for it) of a rim is the distance from the center line of the rim to the surface where the rim mounts to the vehicle. By center line we mean the absolute center of the rim, if we would cut the rim in two equal sizes regarding the width. The wheel offset can be either positive or negative, depending on if the distance is positive towards the outer side of the rim or negative (the contact surface is closer toward the inner side of the rim than the center line is). Almost all wheels are positive or at least zero (which means they have an ET-measurement of zero / 0).
The wheel offset is the insertion depth of the mounting surface of the rim from the center of the rim width.
We know that the above description ain't that simple to picture in the head, so we're also going to describe it with photos and illustrations! Let's start with the mounting surface of the rim, as you can see on the image below it is the surface where the rim meets the wheel hub.
If we were to do a manual measurement of the wheel offset, we need to start by measuring the rim width, the totalt width of the rim material, not the tire width which is included in the image below (sorry for that, didn't have any rim without a tire at this moment). After we have the width measurement we divide it in two, which means we get the center measurement of the rim, the absolute center of the wheel. In the wheel below, we measured the rim width to be 220 mm and therefore the center is at 110 mm.
Now when we know the total rim width and of course center line of the rim, all we need to do is measure the insertion depth (wheel offset, ET). For that we will use a wooden planket which we cut to fit the rim diameter. The plank should not go over the tire because the tire is wider than the rim in this case, and most cases.
With the wooden plank in place we bring out the measuring tape. The distance from the outer edge of the rim to the mounting surface is 150 mm as you can see below.
Now we have all we need to determine the wheel offset, total rim width is 220 mm which makes the center of the rim 110 mm. The center line of the rim is as previously explained where the ET is measured from, so we need to take away 110 mm from the 150 mm measurement which leaves us to 40 mm positive wheel offset! But if the distance from the outer edge where to be, lets say 100 mm, the ET would be negative -10 mm (100 mm - 110 mm = -10 mm).
As this particular rim already have the rim specifications stamped on the inside of on of the wheel spokes, it was kind of unnecessary to measure it manually if it weren't for this blog post, but as you can see below it confirms the method used above, although to be precise the offset is 40,48 millimeter.
You may have noticed that the stamping on the inside of the wheel says 20 inch x 7,5J X40,48, this means the rim diameter is 20 inch, the rim width is 7,5 inch and the offset is 40,48, but hey, lets wait a minute, then how can the rim width be 220 mm as we wrote above when it says 7,5 inch (190,5 mm) on the stamping? That i because the 190,5 mm measurement is the width where the tire mounts, within the aluminum that keeps the tire in place sideways, the total rim width, edge to edge, is 220 mm.
A positive ET mounts the wheel closer to the car while a negative it pushes the wheel further from the car, outside of the fender.
The wheel above actually goes inside of the fender but the angle of how the photo were taken makes it looks to stand out.
Here we have two illustrations from the main website, to further illustrate what a wheel offset is, we start with a animated gif.
The second image below is a static illustration, sorry for the smaller size though, it was made to fit to the facts section at Bolt-pattern.com.
So, now you know (hopefully) what a wheel offset / ET measurement is, otherwise we certainly are a lousy teacher to say the least =) In that case, shout out your question in a email or at our Facebook-page, thanks!
Hope you've enjoyed the guide, see you next time!