Offsets and backspacing are very important as far as how the wheels will fit on your car, it is somewhat complicated but if you spend 20mins to understand it, you'll be fine.
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.
In another word, if your 3pcs ACS wheels was originally 17x7 et 30, by changing the outer barrels to a wider size, you will decrease the offset of the wheels since the centre of the wheel has moved towards the mounting surface.
here's a pic to explain:
Back spacing and offset go together like weed and hookers (may be not), but you cannot calculate your new 3pcs wheels' offset without backspacing, so what is backspacing?
backspacing is simply the measurement of the inner lips to the mounting surface, this measurement will tell you if your new M3 wheels going to rub the coilover shocks or not.
The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace.
What you need to calculate your wheel's offset
So you are wondering why in the world I focus on you need to know your backspacing before you can calculate your new offset, well, you don't want to spend $4k on a set of 18" OZ Futuras with custom barrels then found out they poke out 1" pass the fenders right?
To calculate offset you'll need the following measurements:
-Wheel Center line (outboard flange to inboard flange measurement / 2)
-Wheel center line from Wheel backspace to get offset.
-If backspace is less than the wheel centerline the offset is negative
-If backspace is greater than the wheel centerline the offset is positive
To convert from inches to mm multiply by 25.4
To convert from mm to inches divide by 25.4
Wheel offset caculator
This is extremely helpful to know just how your new wheels will fit compare to the current ones. I will explain how important this can be for people going for the Euro/stretch look.
Backspacing offset conversion chart
This is a quick conversion chart