Just how fast am I actually going?

[gordski]

I've got a GSXR600 fitted with a Garmin Zumo 550. (on a track) I was travleing at 100mph according to my bikes speedo, however, my sat nav said I was actually traveling at 84mph. I know car/bike speedos have to be accurate but never read more than your actual speed, however 16% variance seems high. Just how accurate are sat nav speeds and what speed was I actually doing? Obviuosly I could ride to my local speed camera and pass it at 100, then 14 days later I get a picture with my exact speed, however given the camera is in a 30, I suspect this isn't the best idea!!

[PJR]

That is indeed a big difference, so something is wrong. But in my experience, sat nav speedo's are more accurate than car speedo's. I can't really speak for bikes as i don't ride one, but i'd expect them to be similar to cars in this respect. Usually they will over read by 10% at most. The only issue with sat nav speedo's is that they don't react as fast as a regular speedo does. Still, that doesn't explain your problem. Can you borrow some other sat nav and try that?
Is the bike standard? Any gear ratio changes or wheel/tyre size changes?

P,

[2 sMoKiN bArReLs]

If you are going up a steep hill the Satnav will be inaccurate!

[K4TRV]

gordski said:

I've got a GSXR600 fitted with a Garmin Zumo 550. (on a track) I was travleing at 100mph according to my bikes speedo, however, my sat nav said I was actually traveling at 84mph. I know car/bike speedos have to be accurate but never read more than your actual speed, however 16% variance seems high. Just how accurate are sat nav speeds and what speed was I actually doing? Obviuosly I could ride to my local speed camera and pass it at 100, then 14 days later I get a picture with my exact speed, however given the camera is in a 30, I suspect this isn't the best idea!!

Your first sentence says it all. "On a track".
The sample time of the SAT Nav receiver is nowhere quick enough for track use. On most roads it is satisfactory, but bikes and quicker cars can get caught (braking & accelerating) out if you read Sat Nav speed rather than speedo. In traffic through speed checks the Zumo will be dead-on.

Ultimately a Sat Nav in steady state, even up long steep hills will be more accurate. Just don't use read-outs on track as reference speeds, 'cause they will be inaccurate.

Trev McM

[mightydquinn]

front tyre depth makes a big differance on a bike.
indicated 200 on a zzr1100 was 187 sat nav.
on new tyres.but they can be very different ride at side of some one else doin 50 and compare.

[blueST]

Have you got non standard sprockets? If you've altered the gearing for better acceleration that will cause your speedo to over read more than normal. Assuming the speedo reads off the gearbox, which I think it does on yours.

Also as already mentioned, if you combine that with going up or down hill, making the sat nav under read, the gap between the two readings could be huge.