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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.
 

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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!
 

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On pretty much any of these old bikes it comes out to reading about 5 miles high at highway speeds. It's usually 2 or 3 miles high down at lower speeds, on a graduated scale sort of.
 

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10%?
 

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Mines dead on with my gps. Must be a problem with the older models. Mines a 2010.
 

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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.
 

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Brent,

Try some more tests. Go 55, 50, 45, etc. on your bike and see what your GPS says. That will tell you your speedometer is off at all speeds and you can adjust accordingly.
 

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Discussion Starter #9
Sounds about right. Your speed should be off proportionally throughout the MPH range.



My 1990 is pretty much right on with my GPS.



Do you have an aftermarket tire on your bike?
My Bike is a 2008 and it has the stock tire on it.
 

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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.
 

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How do you know your GPS is displaying the accurate speed? Are all GPS's equally accurate?



BTW, I'm not questioning anyone's judgement, I'd really like to know.



Mark
 

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How do you know your GPS is displaying the accurate speed? Are all GPS's equally accurate?



BTW, I'm not questioning anyone's judgement, I'd really like to know.



Mark
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.
 

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It's a pretty common problem with Japanese motorcycles. My Vstrom was about 8% low as I recall, and it is a common enough issue that I could buy an aftermarket speedometer face that trued up the readout. Interestingly, the odo is often right on, or very close.
 

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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.
 

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My 1997 is within 2 mph across the board. We'll see what happens when I switch out the front tire from stock to the Kenda, When I get's round to it... will post an update. m.
 

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Your gps is wrong. my tee goes 75mph. Whatever is wrong that tells me that is my speed makes me love the dubba more. How accurate is it in your car? I might be going 55 mph, but if the speedo reads 75 the ride is goood!
 

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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.)
 

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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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