offset refers to the mounting surface of the wheel and how much material there is. Our cars range from positive offset to high positive offset wheels. In other words, usually ET20 to ET50. ET50 is more A3/A6. The smaller the number, the more aggressively ‘flushed out’ the wheel will be.
The number is a measurement in millimetres. There are 25.4 millimetres in an inch for perspective.
So a wheel that is ET 20 vs. a wheel that is ET50 will mean a 30mm difference in how aggressive or ‘flushed out’ the wheel will look. 30 mm is a little over an inch…and that’s a ton when it comes to wheel offset.
You can’t forget that wheel width also has to be accounted for. An 8.0 wheel vs. an 8.5 wheel will look different, so you have to account for the change in wheel width in your search. 0.5" = 12.5 mm. Now, that 12.5mm is not entirely on the outside (i.e. it doesn’t flush out the wheel entirely). Rather it’s split on the inside and outside of the wheel. So an increase of 0.5" in width is the equivalent of one half of 12.5 mm of offset, therefore about 6mm.
So, an example. Let's look at the B6/7 S4.
Stock, 18x8.0 wheels ET45. These look a bit tucked. 18x8.5 ET 35 looks pretty awesome. What's the difference? Let's do the math
width change - 0.5" a.k.a 12.5 mm, but remember that’s 6.25mm of outside width, or more flushness
offset change - 10mm
total change - 16.25mm more flush…that’s about 9/16"
That’s a big change, but the stockers are pretty tucky, and the 16mm change is just about perfect. Much more and it could potentially start to poke.
Now let’s look at the guys who rock B7 RS4 wheels on their B6/7 A4/S4
stock S4 18x8.0 ET45
stock RS4 19x9.0 ET29
width difference 1.0" a.k.a. 25.4 mm, which is 12.7mm of outside width, or more flushness
offset difference 16mm
total difference = about 29 mm
You just need to look at a B6/7 S4 to know that 29mm of added flushness is going to result in a stupid looking poke setup that can only be cured by running ridiculous stretched tires and ricer style, performance killing camber.
But I digress…
So looking at your B6 A4 example vs. B5 RS4 wheels, it’s going to be dramatic too.
17x7.5 A4 sport wheels ET45 (is that right? I’m guessing because you wrote it)
18x8.5 B5 RS4 wheels ET20
width difference = 1", or 25.4mm, which is 12.7mm of outside width, or extra flushed-out-ness
offset difference = 25mm (big)
total difference = 38mm
This is massive. This is 1.5" basically, so unless your wheels are CRAZY tucked, this might look silly. So best be careful.
p.s. spacers are a topic that come up a lot. People buy 5 or 10mm spacers to flush out a wheel setup a bit. This is exactly the same as an offset difference. You’re just adding ‘material’ that will result in the wheel mounting up flushed out a bit more. As you can see a 10mm spacer is a big change. That should give you an idea how crazy a 29 or 38mm difference between two wheels would be in our examples…they’re the equivalent of running MASSIVE thick spacers.
