Motorcycle Repair: 2006 Honda Rebel CMX250C Speedometer issues, honda rebel, speed reader

Question
I recently purchased a used 2006 Rebel 250cc. It's my first bike, so it took me a week or so to notice that the speedo is off. I was out riding with someone a week or so ago, and he asked me if I was afraid of riding because I was going roughly 5mph under the limit all of the time. We rode side by side at 45mph (40mph) and compared our readings. I also rode past a roadside speed reader that confirmed that the speedo is off. It's also affecting the mileage reading. I rode from a store to my home on the Rebel and in my truck, and the Rebel (2.5) says it's .8 miles farther than the truck does (1.7). Can I fix this myself or do I have to take it into a dealer?

Answer
Hey Dave,
I dont really see how this would change the miles it read's. But has tire size every been changed?  and also .8 of mile how many turn's did you make a bike is always going to count more or less miles then a car..nature of law...But yea a dealer are a bigger local shop should be able to recalabrate. your speed