I also have Z4s...well a Z3 on the front, (didnt wear as much as the rear) so I only replaced the rear with a Z4, seeing that they dont make Z3s anymore.
I didnt get much out of my Z3s...maybe 7,000 before the belts started showing. So far the Z4 has been an affordable, high performance tire. It has never come close to letting loose. I also tend to ride at high speeds when possible 90-100. It has handled the wet roads with ease, great handling, quick response. Soft rubber...low miles. Right now I think I have about 4000 miles on the rear, next tire change the front and rear are going to Z4s. THOR
