Just for fun and info, my friend and I did some speed runs with my Garmin Etrax GPS this weekend. Conditions were 65degrees on dry gravel road (Flat)
My machine mods are listed here on my profile
All stock except:
Dial-jets ( I don't know what mains are in it, bought it used like this)
HoleShot Module
25"Mudlites
(Clutch is stock as far as I can tell)
(no pipe, bought used this way)

Results: Top speed was 54.7 mph in 2wd, 4wd N/A Speedo indicated 60mph!!!

Put in new TWIN AIR and Top speed was 55.4mph in 2wd and 56.6 in 4WD 61mph indicated on Speedo
(Does anybody know why 4wd was faster?)

My friends machine
03" P 650 All Stock except:
Twin Air
27"Gators
Results: Top speed was 56.2 mph in 2WD (Speedo indicated 60mph)

Comparson: I whoop his *** 0-30mph by 2-3 lengths, then he creeps and passes me at 45mph

 

maybe somethings wrong with speedo

 

I would guess that the diameter of the aftermarket tires accounts for most of your speedo errors and that it may not have been that accurate in the first place.

 

I'd like to blame the tires, but mine has 25" tires and my speedo is 5mph fast. My friends has 27" tires and his speedo is 4mph fast. If anything I would think the 27's would make the speedo further off. If these two machines are off, its likely there are many out there that are off also. I've seen someone claim 67mph bone stock, thats got to be off. I'd expect 2-3mph off but not 5. I know motorcycle manufactures error to the fast side usually 3-7mph around the speed limit.

 

Good point with the 27s it should be reading slow since they cover more distance per revolution. I wonder if its not the GPS then - I know mine when in the truck/rv takes a little bit to catch up when my speed changes. I just noticed that you were on gravel and that may explain the top speed question between 2wd and 4wd as in 2wd drive it had just enough slip on the gravel to slow it down?

 

sure gravel could be part of it, but just FYI I did maintain top speed for a good 1/4mi. I guess I'll have to redo it on pavement, that would be interesting.

 

(Does anybody know why 4wd was faster?): Depends on a lot of variables, main one is how the tires hook up (weight, tires etc...) in 4wd you don't get that much wheel spin you get more power and in 2wd you hit the $^#U*)* rev limiter so infact 4wd is faster than 2wd.... that's what I find.... more so if you monitor speed on a gps than the odo. The 650 has enough engine power to really cruise in 4wd and even in 2wd there are shafts upfont that it has to spin so it's eating some extra power (not much). My 28 read slow my 25 read about right and my tracks read high. gravel is tough to get real speed on... I find a slight incline is better for gravel. Also I don't think 27 gators are 27's they are 26 (not sure). Your holeshot could be eatting away a little at your top end too, the early ones did effect to end.