Speedo is out by 20kms

I've only had the ZX-9R for a week and noticed (after a few rides to work) that the speed I was doing and what was on the speedo was very different, thought that was strange with all these cars passing me especially the grey/purple haired ones. Anyway asked Julie to drive and I would follow her with the GPS on and do the speed limit in a few different zones and yes .......the speedo is out, thought it was the new tyres but it happened at every speed increment at the same difference (20 kms), so couldn't be that, as my car has different sized tyres than the manufacturer suggests and it goes out a couple of km's more as you go faster.
So my question is would a speed healer work and be the best to get to fix the problem if so what type?
Also does anyone have a gear indicator put on their bike like GIPro X-Type, is it of benefit to the bike and you?
Cheers
Steve
So my question is would a speed healer work and be the best to get to fix the problem if so what type?
Also does anyone have a gear indicator put on their bike like GIPro X-Type, is it of benefit to the bike and you?
Cheers
Steve