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Knowing ur Speedo

3K views 19 replies 15 participants last post by  qwerty 
#1 ·
Today I was riding and had my GPS on my bike and my speedo said 65 and the GPS said 60 so I guess whatever my speedo says I can pretty much take 5 miles an hour off that. Just wondering if anyone else's bike does the same.
 
#2 ·
Mine did the same thing, might have even been more. Used to wonder why traffic was piling up behind me in town when I was going the 'allowable' over the speed limit.



That's one advantage of switching to a Vapor or other accessory speedo, you can set it exact. Afterwards the top speed doesn't sound as impressive though!
 
#7 ·
A after market tyre does not make 10% difference.

The tyre must be 10% smaller (diameter) for this difference.



I know it from all of my cars (VW Golf IV, Ford Mondeo/Contour, Ford Escort V and VI) and bikes (TW125, TW200, MZ Mastiff 660), that the real speed is 10% lower as the shown speed.
 
#11 ·
Just for comparison purposes; my 2001 TW was dead accurate at any speed. 35 at 35, 45 at 45 and 55 at 55 with stock tires. When I put TW203/204 road tires on my bike I was doing 53 at an indicated 55, about 4% off.
 
#14 ·
All devices are accurate within a certain percentage. As an example, if your speedometer is correct to +/- 2%, gps +/- 1.5% and you have a front tire that is 1 inch smaller or larger than stock then there is quite a bit of room for error. Next time you see a policeman conducting a radar or laser speed enforcement campaign on the side of the road, stop and ask if he will verify your speedometer reading as you approach at a predetermined speed. Afterwards you can compare your gps to the known speed of the radar/laser . Keeping in mind these units are also only correct to within a certain percentage as well.
 
#16 ·
There are a lot of radar signs (school zone) and radar trailers around here (Atlanta GA).



Between the radar and my GPS, my 07 seems to be a shade less than 10% off.
 
#19 ·
I was able to test with a good radar today, got +/- 1 mph @ 25 mph and at 30 mph. Now, this is a bit subjective, as vibration on asphalt makes it a little tricky to hold flat speed, I made four passes, & believe it's pretty close. m. (Stock 1997 Ca. model w/stock tires, at just over 1000 miles on the bike.)
 
#20 ·
Unless you have a calibrated speedometer, it is probably off a bit. 5mph off at an indicated 65mph is about 8%. Typical. Tdub is a couple mph slower than the speedo indicates from 0 to 40, then closes up to dead on at 65-70, then is 84mph at an indicated 80. TW203.
 
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