15x12W seems like it's a little short to me, but with it being a W it is probably OK. What kind of RPM's are you turning? As Troy points out often, the full-throttle RPMs should be the determining factor. I can't remember what the range is exactly for the 140DZ, but I think it's somewhere between 8400 and 9200. When I first started running the 140DZ I was using a 17x12. I recently dropped down to the 17x10 in order to slow my airplane down.