Title kinda says it all... does anyone know how accurate the stock speedo, and tach for that matter are?

last I heard teh speedo reads about 10% fast.

matt

Quote from: Jascolast I heard teh speedo reads about 10% fast.

matt

Yep, my 2001 model showed 70 and was only doing 63 by my GPS.  50 shown was an actual 45.

http://www.bikepower.net/gs500e-power/modifications/appearance-related/speedometer/speedometerpage1.htm

Here's a chart of Readout/actual speed

Yup, overstate about 10% i believe that in some countries this is a requirement imposed on manufacturers and/or importers of vehicles.

Mine seems very close to being right-on.  I found a nice section of road with mile markers, did my best to keep it at 60mph, looked at the seconds on my watch as I passed the first marker and passed the mile mark right at 60 seconds.  10% off would have gotten me there 6 seconds later.  

I followed that link for a digital speedo, I noticed he put it on the rear tire.  You are aware that as the rear tire wears the overall circumference will change?  Probably quite a bit of an indicated speed change.  I know I've lost at least 1/4 inch of tread.  That would make my overall diameter 1/2 inch smaller.  

I did some quick math with round numbers, not exactly what's on the GS...  At 19" diameter the wheel makes 88.45 revolutions in a mile.  At 19.5" diameter it's 86.19.  That is a 2.5% difference.  At 60mph it's 1.5 mph too high.  Not as significant as I first thought.

Is there possibly another alternative to using a bicycle speedometer?  One that tied in to the chasis electrical system would be nice, also one that had a backlight feature would be very helpful for night riding.  Anyways, thanks for all the help guys.