I've been playing with Autocad 2002 making some simple drawings looking into swapping staggered stock 17" 105s with 225/45 and 255/40 for staggered 18s with 245/40 and 275/35
Red lines are the outline of the tires
Magenta lines are the outline of the wheels
Yellow lines are just working lines I used to establish track width, which also allowed me to establish the approximate hub to hub dimensions using stock specs for track width and wheel ET. Also use them to visualize the differences in clearance between different tires and wheels.
Tire specs I used were Nitto Motivo
Wheel specs were for Voxx Maglia 18x8.5 ET40 and 18x9.5 ET40
Zip file contains my simple CAD drawings if someone wants to play with them. I'm not an engineer or draftsman. Just had enough schooling in Autocad almost 18 years ago to be dangerous LOL.
Z4 stggr tire and wheel drawings.zip
Wheels are 0.023" closer to the inner fender and 0.523" closer to the outside of the fender.
Tire sidewall is 0.117" closer to the inner fender and 0.663" closer to the outside of the fender
Tire diameter is 0.75" larger which is 0.375" (3/8") larger from the center of the wheel.
Overall tire width increases 0.78"
Front track width increases 0.546"
Wheels are 0.11" closer to the inner fender and 0.89" closer to the outside of the fender.
Tire sidewall is 0.17" FURTHER AWAY
from the inner fender and 0.61" closer to the outside of the fender
Tire diameter is 0.59" larger which is 0.295" (just under 5/16") larger from the center of the wheel.
Overall tire width increases 0.44"
Rear track width increases 0.78"
I think this setup could work rather well without spacers. It's what I'm planning to try soon when I gather up a little more "Momma don't know" play money.