Wheel offset calculator - might be useful to those of you wanting to change wheels

Found a nice website (http://marksink.com/tire_wheel_offset/offset.html) to compare different wheels. Wheel sizes and offsets was a particularly confusing area to a non tech savvy person like myself. I found this simple calculator useful to penetrate the haze.

It shows the final result: i.e. how does the new wheel sit in relation to the old wheel.

Taking my 1991 348 as the guinea pig, this is how the most common upgrade options pan out:

1991 348: Baseline
F: 7.5"X17" with +49mm offset
R: 9"X17" with +68mm offset

Upgrade to 355 wheels:
Front 18 x 7.5" with +32mm offset
Result:
The clearance from stut housing to the inside of the wheel will be 17mm MORE
The outside edge of the wheel will EXTEND an extra 17mm
--> effectively it is like adding a 17mm spacer to the 348 wheel

Rear 18 x 10" with +77.9 offset
Result:
The clearance from stut housing to the inside of the wheel will be 22mm LESS
The outside edge of the wheel will EXTEND an extra 4mm
--> The wheel will barely extend beyond the stock 348 wheel but it sits too close to the hub and suspension pieces. Makes sense to add a 25mm spacer to bring the wheel out.

So if I were to put a 355 wheel set on my car, I would buy a 25mm spacer for the rear (type 2 fixing). The front wheel is okay as is (there is room at the wheel well).

Upgrade to 360 wheels (starfish):
Front 18 x 7.5" with +31.5mm offset
Result:
The clearance from stut housing to the inside of the wheel will be 18mm MORE
The outside edge of the wheel will EXTEND an extra 18mm
--> effectively it is like adding a 18mm spacer to the 348 wheel

Rear 18 x 10" with +39mm offset
Result:
The clearance from stut housing to the inside of the wheel will be 16mm MORE
The outside edge of the wheel will EXTEND an extra 42mm
--> The wheel will extend well beyond the stock 348 wheel. Will look fugly.

A 360 wheel upgrade does not make sense to me unless you machine down the rear wheel. The front wheel should fit fine. Note: 360 wheels will fit much better on the 348 Spider / SS because of the reduced offset in the rear.