The wheel offsets for the Z8 are 20mm front, 22mm rear.
Here is a list of tire sizes suitable for BMW Z8.
Front stock tire size:
245/45/18 339mm in radius
Front upgrade tire size:
255/40/18 331mm
245/40/19 339mm
255/35/19 331mm
265/40/19 - GM
245/35/20 340mm
255/35/20 343mm
Rear stock tire size:
275/40/18 339mm in radius
Rear upgrade tire size:
285/40/18 343mm
295/35/18 332mm
275/35/19 338mm
285/35/19 341mm
295/30/19 330mm
285/30/20 340mm
The chart suggests that the best front and rear combination is 245/40/19 front / 275/35/19 or 285/35/19 rear in 19", however GM has been running 265/40/19 - 285/35/19 and is very keen on that combo as the wider front footprint gives increased grip and neutralizes understeer.
This is a weight comparison chart for some of the common tire/wheel combos
wheel...................tires...............................front............................................rear
Stock.....................Bridgestone RE040r/f........62lbs/28.1k - 245/45/18..................67lbs/30.4k - 275/40/18
rims alone..............18 x 8 / 18 x 9................29lbs/13.1k...................................31lbs/14.1k
Alternative tires on stock OE 18" rims
Stock.....................Bridgestone RE-11...........61lbs - 245/45/18...........................66lbs - 275/40/18
Stock.....................MPSport 2......................54.5 lbs - 245/45/18........................58.5 lbs - 275/40/18
19"
BBS RGR 19"...........Michelin PS2....................46lbs/20.8k - 265/35/19..................50lbs/19.5k - 285/35/19
BBS RGR 19"...........Yokohama Advan Sport.....49lbs/22.2k - 265/35/19..................53lbs/23.6k - 285/35/19
BMW Style 166 19"...Conti Sport Contact 2........53 lbs/24k - 255/40/19....................59.5 lbs/27k - 285/35/19
rims alone..............19 x 8.5 / 19 x 9.5............26lbs/11.8k...................................29 lbs/13.1k
20"
BBS RS-GT 20"........Michelin PS2....................48lbs/21.8k - 255/40/20..................52lbs/23.6k - 285/35/20
Hamann 20"............Michelin PS2....................61lbs/27.7k - 255/35/20..................63lbs/28.6k - 285/30/20
rims alone..............20 x 9 / 20 x 10.5............31lbs/14.1k...................................36 lbs/16.3k
BMW M Sport 20".....Pielli P Zero.....................61lbs/27.7k - 255/35/20..................66lbs/30k - 295/30/20
Alpina Rims 20".......Michelin PS2.....................57.2 lbs/25.9k - 255/35/20..............58.7lbs/26.6k - 285/30/20
rims alone..............20 x 9 / 20 x 10................31.6lbs/14.3k................................32.5 lbs/16k