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Discussion Starter · #1 ·
OK so I'm noticing that depending on what device you use to monitor spped, you'll get a different reading.

A top speed video on da toob, showed a guy doing 103MPH according to his GPS, but the 300's speedometer read like 110MPH.

Just today I watched a dyno run (Garth285) and he says the dyno printed 113MPH while the bike said 121MPH.

So is the bike's speedo not reliable or are the GPS and Dyno wrong?
 

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I was actually researching this today. Speedo's can be off by 10% and generally are apparently. not sure why. So if you are reading 90mph, you are going 81... somebody said to make you think you're going faster than you are, or for when you make mods to your bike you won't be going faster than it displays (such as changing sprockets).
 

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the majority of bikes/cars off the show room floor will be travelling slightly slower then the speed indicated on the speedometer, they have been for years.

its a good idea, it helps to keep your licence longer, you can verify this with a gps. it was mentioned in another thread here on the forum if you go from your stock sprocket of 14 to 15 the speedometer will be accurate
 

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I was actually researching this today. Speedo's can be off by 10% and generally are apparently. not sure why. So if you are reading 90mph, you are going 81... somebody said to make you think you're going faster than you are, or for when you make mods to your bike you won't be going faster than it displays (such as changing sprockets).
All manufacturers do this. They make the speedo read higher than you are actually traveling. The discrepancy becomes larger the faster you travel. This is to keep people from traveling too fast in the pursuit of a certain speed.

You can buy something called a 'speedo healer' to fix this.
 

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Discussion Starter · #6 ·
The troopers in my state have an unwritten rule.
"9 you're fine, 10 you're mine"

They generally tolerate anything 9 miles or less above posted speed limit. I guess it makes sense because if I'm doing 64 in a 55 according to my speedo, I'm really doing the speed limit according to the cops radar.
 

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Yeah speedo's are way off but its with every speedo I think, not just in bikes. My car speedo reads slow, so when i am going 70, I am actually going 73.

My scooter is horribly inaccurate haha like 12 mph off... dont know about the ninja yet, havent gotten a gps on there, but I assume its gatta read fast by a little 10% prob.

I do have a +1 front sprocket.. so it may be ok..? who knows.

I would trust a gps over the above any day. even a dyno, just because the dyno machines may not be calibrated for speed.
 

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14/40 sprocket combo is great on the Ninja 300 (stock front, 40t rear) and will correct the speedo-reading. First gear becomes more usable, but the entire gear range feels great too. +1 in front makes the gearing feel too tall (from my experience).
 

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You also have the long federal law that makes it illegal for a speedometer to read under. For example, if you were going 50 but the speedometer read 49. That would put the manufacturer in huge trouble and is a very hefty fine. This is why they usually put in a large bias gap the other way to make it so that the speedometer always read higher than what you were going to eliminate there every being the chance of the speedometer reading under actual. The law doesn't give any limits or says the speedometer can't read higher than actual. Just cannot read under. It's just another check your ass card they play to keep the fines away. Also, most consumer GPS systems aren't as accurate as you think you are. The figure of merit for most consumer based GPS is around 5-6 and can be 100'+ off which would make your relative speed off. Hell, military units in aircraft that cost over a million dollars generally get a figure of merit of around 1(best) to around 3 average and that's still around 25' off or so. When riding around as well, the speedometer on the bike only measures shaft speed and not wheel speed. So it doesn't take into account at all the increased drag as you go faster. Which is why the discrepancy increases the faster you go as you really start seeing a lot more wheel slip from aerodynamic drag as the vehicle goes faster.
 

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Reviving an old thread.
I havnt tested mine, but I feel like it is WAY OFF.
One of the local roads is 45mph, where people usually go 52-58.
I'm going more like 62-65 to stay with the flow.

My dads a police officer, I may use his patrol car's radar to see how off it really is.
 

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Doubt it is off that much. Although I did notice today on the Interstate I was just creeping along and the Speedo read a solid 15 over the speed limit (60), I was slowly passing other cars. And I happened to look down and 75 sure looked a lot more like 65. I even said aloud- "Is it me or does the road really not look like it is coming at me at 75" (although I had just been going 100+ 10 minutes prior) :D
 
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