speedo/odo question

According to my searches and from my own first hand experience, the speedometer is off by about 2-3 miles per hour around 60mph. Theoretically if the speedometer is that far off, wouldn't my odometer be off too? Just for my own confirmation I used my own GPS to see if indeed it was and discovered something alarming? Even though the speedometer is off, the odometer seems to be spot on? Truthfully, I'm not worried about my speedometer being off but more worried that my odometer was giving a false reading. Since I decided to sign up for Fuelly, I wanted to make sure I was posting accurate readings. Is it possible for the speedometer to be off but the odometer be pretty accurate?