Speedometer Error

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It has been routinely noticed that the stock speedometer is not precise. The error can be up to 10%, which can be tested by any speed measuring device put by police on the roads for your convenience, or by a modern GPS. This error is not limited to Kawasaki, or even motorcycle, speedometers. Automobile manufacturers routinely install speedos that read fast, most likely to avoid being sued if their customers get speeding tickets.

There are several ways to remedy the situation:

  • Play with the speed calculator to see the difference between actual and indicated speed.
  • Do the math in your head. Just subtract 10% of your indicated speed. So, if the speedometer shows 60mph, your actual speed is 54. It's real easy.
  • Change your front tire to a bigger one. A 100/90-16 front tire will be just about right for speed and odometer reading.
  • Install a bicycle computer and calibrate it properly.
  • Install a GPS for real accuracy. That's not covered in FAQ, but doing a search on 'GPS' will give lots of choices.

Odometer error

Short answer: There isn't any unless you're really, really picky. If you are, get a GPS.

The odometer and the speedometer are driven by two separate systems in the speedometer head. The odometer uses a rigid gearing system driven by the speedometer cable, so there's no way for the ratio between the front tire and the odometer number wheels to change. It's fixed. The only variable is the diameter of the front tire. The speedometer needle is driven using a spinning magnet inside a steel bell resisted by a coil spring, so there's plenty of room for errors in that system.

If you dismantle an EX250F speedometer and count the gear teeth, here is what you'll find:

  • The speedometer hub gear ratio is 23:9 - For every 23 turns of the front wheel, the speedo cable turns 9 times.
  • The first gear set inside the speedo head has a ratio of 10:1 - For every 10 turns of the speedo cable, the cross shaft spins 1 time.
  • The second gear set has a ratio of 16:1, so every 16 turns of the cross shaft turns the longitudinal shaft 1 time.
  • The last gear set ratio is 14:1 - Every 14 rotations of the longitudinal shaft turns the 10ths wheel of the odometer 1 time.
  • If you do the math, the overall ratio between the wheel and odometer is (14x16x10x9)/23, or 876.5:1.

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Tire size variables: The stock front tire for the 250 Ninja F is 100-80/16. The nominal diameter is 22.30" and calculated theoretical circumference is 70.06". This will vary somewhat by manufacturer and tire model. The stock size tire rotates 904.37 times in one mile. This will make the odometer read 1.03 miles for each mile traveled, roughly 3 percent error over. A common replacement size for the front tire is 100/90-16, which has a diameter of 23.09" and a calculated circumference of 72.53", again variable by tire model. This size rotates 873.58 times per mile and will make the odometer read .997 miles for each mile traveled, about 1/3 of a percent under actual miles. In both cases, this is a relatively small error.

If your definition of accurate is zero percent error, then yes, there is some error in the odometer reading. But in the real world, that's just not possible. Every little thing will affect odometer accuracy, including tire wear, road temperature, tire pressure, and ambient temperature. For everyday purposes the Ninjette's odometer accuracy, though not a perfect 100 percent, can be considered to be spot on. More importantly, the ratio of front wheel turns to odometer wheel turns can never change or vary, unless gear teeth are physically stripped or broken. If that's the case, then the typical result is a complete failure to function rather than a decrease in accuracy.

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