Which mode are you using on the G80 when comparing swing speeds? Also are you hitting a ball when looking at club head speed? ...or are you not using a ball?
Make sure that the spacing between the ball/club at address is proper, and level with the surface being hit off of. The G80 should be a foot back, and if not training club speed with a ball, be sure the club face is coming through where the ball location would normally be.
If you continue to have concerns, it would be suggested to reach out to Garmin Support to discuss the concern live, so one of our experts can get more info from you to gauge what could be causing the discrepancy.
Also make sure that the G80 is up to date on the most current version of software. Software version 4.10 is the most current for the Approach G80, so make sure the G80 is not on a lower version (like version 2.70) for example.
