How accurate are GPS speedo's?

I'm looking to get a speedo heeler for my ZX10R as I'm about to change the gearing on it and want the speedo (it's a rocketship anyway) and odometer to read correctly. One of the methods of calibration is to use something you know can reliably identify 60mph.
So, that's the wife in the Lexus ruled out then.

I've got a GPS-mount on the 10R - will my Garmin Zumo actually be that accurate?? Reason I ask is that when bombing along on the 10R, or even in my van (race bike van really) with the Garmin mounted in it, when the speedo has read like 80mph the GPS has been way off that, yet our speed relative to other traffic would suggest the higher speed So I don't trust it that much.

Should I?

from my experience all sprocket-driven speedo's lie... my Blade would display 299km/h, which is max it could display (it would go faster), and gps would show 270 and up.
when riding with my buddy on gsxr1000, SD speedo reads 200km/h, his shows ~225, GPS shows 198-199.

The Garmin should be as near 100% accurate as it's possible to get.....but.... it is measuring the distance travelled in the time since it last updated rather than measuring in real time so is showing an average speed not actual. The only way to get it spot on is to ride at a constant speed for a few seconds and see what the Zumo says.

You should then be able to calibrate your Yellow Box from the readings then go out and try again to make sure you got it right.

And don't forget your van's speedo is quite likely to be out as much as 10% if it's on standard tyres - more if they are a different size and/or worn.

What about those track GPS units I see for mega bucks?...
They seem to tell you everything, how well to they work? anyone had a whirl with one?