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:
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:
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.