I am assuming Garmin did tests on the zumo 665 and the GXM 40 works underneath the Harley outer fairing. When I recently spoke to Garmin on the phone about this issue, the representative actually said he'd recommend mounting the antenna on top of the gas tank. I told him if that's what's required, they aren't going to sell too many of these to Harley owners. He said it needs to be level and a clear view of the sky and did not know if it would work through the outer fairing. I'm assuming it will since the current small XM antenna on our CVOs does and I assume that's how it works with the Zumo 550 (even though that antenna doesn't get weather). He said the GXM 40 is about 3 inch diameter, so I think it will be fine on the spot the current XM antenna sits.