wheel fitment: rough sketch drawing of example of size and offset for audi s3

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i recently had some questions via messaging about what would fit. i am a huge proponent of teaching those how to fish rather than giving them the fish. in this case, the answer. im doing these example calculations for fun and im not doing this mod just yet. i like to sit on ideas for a while before doing them because new ideas or new parts may come my way to help make a project better. im a modder and i personally dont like to simply pay others to do the work for me, rather i wouldnt mind paying them to teach me. so the satisfaction is the work itself and seeing if the final product becomes what i imagined. for those who are into getting your hands dirty then feel free to follow my adventure.

in this project i plan on showing those of you how to do your own measurements, instead of asking if it fits. for the most part i used a simple measurement tool. a ruler. it should be a wooden/plastic ruler. a tape ruler is hard to keep straight in one hand and a metal ruler can damage your paint if you slipped. a wooden/plastic one is more forgiving if you slipped on the body panel. for my case i have a wooden one and a metal one on hand. of course ill be introducing more tools as they will help out more but the basic part is just a ruler.

firstly im considering two sizes of wheels for myself. use my rough examples to learn from and then do your own calculations. 18x10 et35 and 18x9.5 et45.
we will be selecting a style of wheel we like first, then calculating the wheels itself to see if they fit the car. if they dont then i try to make it fit. why the wheel first, because if i like a certain style of wheel then i will try to make these wheels fit on my car. most of the time you will be going this route.
as opposed to, if you know the car's wheel specs most likely you will not like the wheels with those specs.
welcome to modding.

with my ruler and a scratch paper i drew up this rough example. i tried to draw this to actual scale (caveat, some numbers are rounded) so i can physically see how much larger each width is going to be vs stock. i always like to compare to stock as its something i know what it looks like and its tangible because i have it on the car now. the bottom one in my sketch is the stock wheel spec broken down. here im trying to see if the back space is deep enough and if the front facing side has enough before hitting the fender.
my drawing is crude and might take a moment to get the idea of what im showing.

now that we have that portion done, the next step is to go to the car and remove one front and rear wheel to take measurements. why? because the front fender and rear fender spacing will be different.
also another thing to remember is that the each suspension setup is going to be different. so, when you install different suspension and/or a camber kit youll need to remeasure and when you have these items adjusted every time they will also change the measurements. this is a good thing as those will allow you to install wheels that will normally not fit. so use those also as a tool for fitment.

going to the car and doing the next step will be another time.