Speedometer out of calibration?

Hi guys, riding my '04 TW around I've found that every single street side radar sign that tells you your speed is saying that I'm going 3-4 mph less than what my speedo reads. So if I'm going 30 mph on my gauge, then I'll actually be doing 26 or 27. Maybe that is why I'm always being tailgated. Anyway, does anyone know if there is a way to adjust the speedometer to read correctly? I know on my scooter a ******* way of doing it was taking apart the speedo and moving the needle while holding onto the shaft with needle nose pliers. Is there a more elegant solution? Thanks!